Add a backups facade to the state API server, along with a Create method.

This patch adds the new facade for backups, structured to support the various backups-related API methods that will be added.  An initial method, "Create", is also added which creates a new backup and returns the resulting metadata.  The API client side will be handled in a subsequent PR (see https://github.com/ericsnowcurrently/juju/pull/5).
Use errors.Annotate(err, "contextual info") or errors.Trace(err) if there is no useful info to include.