The Branch protection API has been radically changed by GitHub, add new
methods to Branch reflecting it. Branch protection methods are now
called on the Branch object itself, rather than awkwardly hanging off
the Repository object and requiring branch names to be passed in.
This adds an over-arching methods to get, edit and remove protection
entirely, as well as fine-grained methods for getting, setting and
dropping certain aspects of branch protection, as well as three new
objects to encompass querying those aspects.
This also destroys Repository.protect_branch(), the endpoint has been
removed. The old-style protection attributes on Branch have also been
removed to force users onto the new API, since they are still sent, but
no longer populated.
Fixes#586
