WIP - Add Bitbucket support to git-review

@b4mboo this is a work-in-progress pull request and not ready to be merged yet.

Current status:

1. Bitbucket support is added for all git-review commands. A minor thing to notice is that BitBucket doesn't have the concept of "closing" a pull request. A request is either "accepted/merged" or "declined". In this case, we can consider the "merged" state as "closed".

2. Current tests are all passing with minor changes.

I'm working on separating/rebasing this long list of commits into different pull requests so that each is easy to review and merge incrementally.