BitBucket integration

This builds on the excellent contribution of @MufriA in this PR: https://github.com/crashlytics/crashlytics-services/pull/31 to add BitBucket integration with Crashlytics.  Stay tuned for updated screenshots and status as we make progress on integrating this.
Similar to other integrations where we escalate info returned from the third party service, it'd be good to verify the error message contents contain the response body/code.