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
My bad, I just remembered you mentioned https://github.com/ericsnowcurrently/juju/pull/4 on #juju-dev. ```Backups interface``` is used for mocking, good.