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
removed Action trigger from worker ModeHookError

This patches a bug in the uniter which can result in ModeHookError being
cleared by a successful Action run on the impacted unit.  Actions can no
longer be run in ModeHookError.

This will be resolved after the Context refactor on worker.