Expose a top-level abstraction for all the backups functionality.

This patch adds a top-level backups abstraction.  It also exposes the backup creation functionality through a method.

The PR that will follow this one:
  https://github.com/ericsnowcurrently/juju/pull/4
Manually dereferencing a pointer is almost always a sign that something is wrong.  IIRC, the ConnInfo doesn't really need to be a pointer anyway (it's not a large value... and we aren't going to be passing a lot of them around regardless), so maybe just pass it in as a value?