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
As someone new to the backups implementation, I think backups.Create *is* a nice abstraction over the messy details of dumping mongo and gathering a bunch of files into an archive. I also agree with your point about adding other related methods to it per the ref doc above.

However, I'd like to see the refactoring actually used in the same pull request. I'm also a little skeptical about the need to separating the ```Backups interface``` from the ```backups struct``` -- unless you're going to mock out the implementation to consumers of this interface. Which could be a nice way to improve the juju command tests for backup.