Introduce RemoteCollection

This lets us look up remotes by name, which is not possible by just
returning the list of remotes.

Move remote creation to Repostiory.remotes.create() and keep the old
Repository.create_remote() for compatibility, delegating to this new
way.

Existing code should keep working, but this moves us towards what we'd
need for a better interface in 0.22 which makes remote renaming and
deleting work with a name rather than an instance and would make sense
to exist as part of an Remote.remotes object.