* 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`
