* Marking database tests as slow

* Marking url command tests as slow

* Marking every test that uses database as slow

* Marking tests that import files as slow

* Marking gpg tests as slow

* Marking all versions and one list tests as slow

* Added more markers to unit tests + cli option to skip slow tests

Following a discussion with Axel, the generic 'slowtest' marker has been
split into 'db', 'network' and 'maybeslow'. A brief description of the
meaning of each marker has been added to pytest.ini.

A command line option to run only fast tests has been added to
'spack test'

* Don't use classes to group tests together

Reverted grouping tests under a class, as required in the review

* Minor style changes

We moved to a new GitHub org! Now make the code and docs reflect that.

* Add package for staden-io-lib

* Flake8 cleanups

* Massive conversion from Package to AutotoolsPackage

* Forgot to convert p4est to AutotoolsPackage

* Fix typo

* Fix broken link in docs

Htslib was pulled out of samtools and made into a standalone thing.
This commit adds a packag for it, in support of the samtools package.

WIP: I started trying to build gtkplus@3.20, but this package has many more
dependencies than v2 and it requires newer versions of existing packages. This
commit provides updates for 5 packages that are required by GTK+3. This is not
the complete set of changes required for GTK+3.

atk  - move default version from 2.14 -> 2.20.
glib - move default version from 2.42 -> 2.49
     - v2.49 requires pcre+utf as a new dependency.
pcre - if variant +utf is selected, add '--enable-unicode-properties' to the
       configure options.
libepoxy - new package to spack
         - manages OpenGL function pointers.
pango    - move default version from 1.36 -> 1.40

+ Installation of subversion may need to be modified to include python and perl
  bindings. I have provided comments to this end, but the code is not active.

Package repositories now look like this:

    top-level-dir/
        repo.yaml
        packages/
            libelf/
                package.py
            mpich/
                package.py
            ...

This leaves room at the top level for additional metadata, source,
per-repo configs, indexes, etc., and it makes it easy to see that
something is a spack repo (just look for repo.yaml and packages).

- Packages now live in <package_name>/package.py

- spack.packages refactored to use a PackageDB object instead of
  monolithic module.

- Implementation of mock_packages_test.py is greatly simplified

- Added test to exercise install/uninstall code because that wasn't
  covered by existing tests and kept breaking.

Added more test cases for multimethods.  In doing so, (re)discovered that
satisfies() really needs to handle dependencies properly.

Implemented support for dependencies in satisfies, but constrain() now
isn't consistent (as we do not currently constrain deps), so need to
implement that.

Virtual dependency support probably needs some deeper thought.  i.e.,
there is probably an intermediate DAG form that would make the needed
checks easier.  Right now we have to build ProviderIndexes to figure out
how virtual dependencies are set up.  If the vdep were preserved in the DAG,
then we could just check for things like incompatible providers directly.

- Added more tests
- cleaned up spec preorder traversal
- fixed concretization