Added support for querying by tags (#4786)

* Added support to query packages by tags.
    - The querying commands `spack list`, `spack find` and `spack info` have
      been modified to support querying by tags. Tests have been added to
      check that the feature is working correctly under what should be the
      most frequent use cases.

* Refactored Repo class to make insertion of new file caches easier.
    - Added the class FastPackageChecker. This class is a Mapping from
      package names to stat info, that gets memoized for faster access.

    - Extracted the creation of a ProviderIndex to its own factory function.

* Added a cache file for tags.

    - Following what was done for providers, a TagIndex class has been added.
      This class can serialize and deserialize objects from json. Repo and
      RepoPath have a new method 'packages_with_tags', that uses the TagIndex
      to compute a list of package names that have all the tags passed as
      arguments.

      On Ubuntu 14.04 the effect if the cache reduces the time for spack list
      from ~3sec. to ~0.3sec. after the cache has been built.

* Fixed colorization of `spack info`