Adds bitbucket support

This adds backend support for bitbucket repositories.
Missing:
- An interface to add bitbucket projects
- Remove some hardcoded github links (mostly views)

Limitations:
- Bitbucket API only allows fetching 15 commits at a time.
Uses reversible dir.up in migration
Merge pull request #53 from hugopeixoto/bitbucket-support

Adds bitbucket support