Bug concretize external virtuals

* Add new version property to handle joined version numbers

* Add unit test for new joined property

* Add documentation on version.up_to() and version.joined

* patch directive : fixed retrieval from urls fixes #1584

 - add support for 'gz' archives
 - fixed bugs with URL patches
 - updated nwchem

* patch directive : added checksum to UrlPatch

 - refactored classes in patch.py
 - updated nwchem

* patch directive : added caching

* Fixes needed to build gcc on Linux/ppc64le

* flake8 fix.

Things that accessed the cdd package, such as `spack info cdd run
tripped over a buglet in the *cdd* package, causing them to exit with
something like this:

```
Caused by:
TypeError: 'str' object is not callable
  File "/rss/spack/lib/spack/spack/repository.py", line 584, in get
    self._instances[key] = package_class(copy)
  File "/rss/spack/lib/spack/spack/package.py", line 398, in __init__
    f = fs.for_package_version(self, self.version)
  File "/rss/spack/lib/spack/spack/fetch_strategy.py", line 852, in for_package_version
    attrs['url'] = pkg.url_for_version(version)
  File "/rss/spack/var/spack/repos/builtin/packages/cdd/package.py", line 40, in url_for_version
    str(version.dotted()).replace('.', ''))
```

@tgamblin pointed out that `dotted` is a property, not a functin call
and that the parentheses are therefor inappropriate.

This deletes the parentheses.  `spack info cdd` now works for me.

Add package for htop, an interactive text-mode process viewer for
Unix systems.  Think top, with pretty colors and dyanmic bar graphs.

More info [here](https://github.com/hishamhm/htop).

If Spec.dag_hash was called for the first time with a 'length'
specified, the cached hash was truncated. This ensures that the
full hash is cached.

* Removes the extra argument from Package.do_install while maintaining the changes in behavior pulled in #1603

* install : removed -i and -d shorthands (breaks backward compatibility)

* Change ':' to ','

* petsc: require boost for :3.5 only

* petsc: turn off download for c2html and hwloc

* Add package bazel

* Add support for bazel-based builds

* Update the krell institute products to use the latest features of spack for building on cluster platforms.

* Address travis error messages and resubmit the pull request.

* Update the contents of openspeedshop package.py so it passes the flake8 tests.

* Fix flake8 error-whitespack issue in mrnet package.py file.

* Add updates based on spack reviewer feedback.

* More fixes based on comments from reviewers.  Switch using extend to using append, remove additional setting of PATH and LD_LIBRARY_PATH that should not be required due to RPATH.

* More review related changes.  Update MPIOption.append lines and take out xercesc references.

* Create a base options function for common openspeedshop base cmake options to reduce redundencies.

* fix download url for paraver

* added note to remove older version

* Ignore GPG/PGP signatures/checksums when finding available downloads

* Remove duplicated basename

* Add platform-specific configuration scopes.

* Update `spack config` to use the new scope arguments.

* travis : unit tests will run with and without libyaml

* travis : fixed package that are installed

* Workaround for illegal package merging in py-matplotlib/py-basemap

# Conflicts:
#	var/spack/repos/builtin/packages/py-basemap/package.py

* flake8

* flake8

* Be conservative: only apply the namespace package fix for Python >= 3.5.2

* flake8

* Bug fix

* py-macs2: New package

* py-macs2: add py-numpy@1.6: dependency

* Major updates to Contribution Guide

* Grammar changes

* Fix missing/extra backticks

* Rewording, links, and tips added

* petsc: add 3.7.4

* slepc: add 3.7.3

* Made optional CGAL dependencies optional.

* cgal: Added note explaining that the CORE library is not the same as core CGAL functionality.

* Bug fix and flake8

* flake8

* build_environment: allow compilers to set up an environment

* clang: mock up a toolchain directory for xcode

Some projects ignore CC and CXX flags and instead use xcode to find the
toolchain. Clang on Apple should set up the environment properly.
Arguably, every compiler could do this on Apple, but let's see how this
works out just for AppleClang for now.

The Documentation directory is ~1.7G and the excluded platforms add up
to about 7G. Ignoring swift saves another 500M. The resulting Xcode.app
copy is in the 2G range.

* compiler: set member variables early

This is required so that later methods can query things such as the
version of the compiler.

* compiler: support finding the real path of the compiler

On Apple, the /usr/bin compilers are actually wrapping tools themselves
which query xcrun for the currently selected Xcode installation. Pierce
this veil and get the real, full path the to underlying compilers
instead.

* icu4c: install with rpath

On macOS, icu installs with a library ID of the library name. Enabling
rpath makes its ID its full installed path which lets Qt5 link against
it successfully.

* qt: no -no-gtkstyle flag on Qt5 on macOS

* spack list : updated documentation

* spack list : removed space before ':'

This PR add documentation for the `suffixes` and `hash_length` options
for modules.yaml.

This resolves #1416.

* patchelf: Add version

* flake8

* Removed unnecessary url_for_version() function.

* Restored old URL