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).
This will be backups.BackupsAPI when used by other packages, backups.API would be better. Either way, comment should start with the name of the struct.