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
You're right.  I did that for testing at a time when db.NewDumper() returned a *mongoDumper.  However, that's no longer the case.  I just missed fixing this at that time. :)