Bitbucket support

Hey - I had a go at supporting Bitbucket too because we use that quite a bit.

Sadly the npm for bitbucket uses their old v1 API and just execs curl commands against it. I haven't found a newer npm so will probably start writing one, but for now it's what I've got to work with.

Changes/commits not supported for bitbucket yet. That will probably need more functions pulling into the client/provider class.

Also a few minor doc updates.

So, want to merge this in?

Dan