Add Bitbucket support

[b4mboo]

Right now git-review only supports GitHub. It might however be nice to extend the functionality towards allowing other services (i.e. BitBucket).

[b4mboo]

The idea is to basically create a factory pattern that adds an abstraction layer GitServer, whose instance replaces the current occurrences of the GitHub instance. Internally it should pass the requests on to the actual implementation instance. Such that a request for e.g. @git_server.open_requests would query the data from the correct server.

[b4mboo]

@xystushi is creating bucketkit to access BitBucket's API.

[b4mboo]

@xystushi I just found some time to hack on git-review. There is a lot to do, but I think we're on the right path. I'm ripping the old github.rb apart and make this way more object oriented. Once this is done, extending it to BitBucket should be way easier. Take a look at my progress in the development branch and tell me what you think, if you find the time.