Add a backups abstraction.
Add Create() to the backups abstraction.
Use values instead of pointers.
Add missing blank lines before comments.
Drop unnecessary type declaration.
Add tests for failure cases.
Merge pull request #708 from ericsnowcurrently/backups-create

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