Support different versions of the Bitbucket API

[ty1824]

A follow-up from #15 - we should have support for v2.0 of the API. This may be implemented as a separate connection depending on the differences between the versions.

[justinhorton]

As I alluded to in #15, I don't think there's any reason to support multiple API versions for a particular endpoint. I'd be fine using this to switch to v2.0 for Bitbucket Server. Although, the more pressing issue right now is #11.