Cleaner log messages during artifact build
Make artifact.json contain full list of dependencies for each artifact
Implemented garbage collection
Add warning about not using plain 'mv' on profile symlink
BUG virtual dependencies were not supported in gc; just ignore them for now
BUG fix build store unit tests fixture (new ~/.hashdist structure)
Start on debug-mode feature
fixed PS1 environment assumption
Merge remote-tracking branch 'origin/master' into debug

This resolves @certik's keep_build flag into the debug branch.

This also unifies the calling parameters for builder.build as mandatory.

Conflicts:
	hashdist/cli/frontend_cli.py
	hashdist/spec/builder.py
Merge branch 'origin/master' into HEAD

Conflicts:
	hashdist/cli/frontend_cli.py
Added sensible debug defaults to builders

Sensible default arguments for keep_build and debug
Merge pull request #187 from hashdist/ahmadia/debug

Debugging support