added tar.xz support
Merge pull request #115 from mephi42/tar.xz

added tar.xz support
minor doc fixes in core/build_story.py
added robust_rmtree
incorporated suggestions from @certik and @dagss
Aron sucks at control flow
Merge pull request #116 from ahmadia/robust_rmtree

added robust_rmtree
Merge branch 'master' into spec

Conflicts:
	hashdist/core/build_store.py