This moves us entirely (almost) onto the Bitbucket 2.0 API. The old 1.0

API has a number of bugs, which have bit some users, and 2.0 has

recently been expanded to include most of what we need (without the

bugs and with improved error reporting).
We now use the 2.0 API for retrieving branches, commits, checking the

repository, and validating the user. We can't use it yet for fetching

the contents of files, though, as that's still a 1.0-only feature

(though is being tracked upstream by Bitbucket -- see

https://bitbucket.org/site/master/issues/13690/customer-request-api-20-to-include).
One of the advantages of the new API is that payloads can be limited to

the specific fields needed in a client. We now use this to slim down

communication with Bitbucket.
There's also additional bullet-proofing in here for validating that the

repository is of the correct type, based on the choices selected in the

repository form.

Unit tests passed.
Tested with a handful of repositories (Git and Mercurial, and including

one that triggered crashes in the Bitbucket API). I tested that

repository validation, account linking, branch/commit listing, and

file fetching all work.