Add Bazaar support

The support is still quite limited:
  - green if not modified
  - red   if modified (shelved changes are currently considered as changes)
  - Current revno is displayed

TODO:
  - Since bzr is slow to start, refactor liquidprompt to not invoke bzr twice (smarttag and bzr_prompt)
  - Display not pushed changes
Merge pull request #94 from cykl/develop

Add Bazaar support