Simplified compiler finding logic and caching.
import & namespace fixes

- Fix Boost issue reported in SPACK-16
Add more output; don't re-add existing compilers
New spack mirror command, configuration.

- Mirrors now appear in ~/.spackconfig, can be edited in that file.
- spack mirror command allows adding, listing, removing mirrors
	- Also still creates mirror directories.
spack mirror now checksums fetched archives.
Merge pull request #16 in SCALE/spack from features/mirror to develop

# By Todd Gamblin
# Via Todd Gamblin
* commit '3c3f272280c530553322142d9d836c91b1b01137':
  spack mirror now checksums fetched archives.
  New spack mirror command, configuration.
  Add more output; don't re-add existing compilers
Bugfix for mirror command
Updated docs.