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.
Once you think your gem is ready to use, please push it to rubygems.org (`gem push ...`), such that we no longer need to pull from the repository directly.