Add an API facade for backups.
Add tests.
Add missing doc comments.
Wrap returned errors with errors.Trace().
Drop the superfluous "Backups" from some names.
Fix a typo.
impl -> backups.
Add the newMeta() test helper and use it.
Remove unnecessary XXX comments.
Use the more canonical form for an empty value.
multiline -> singleline.
Merge pull request #731 from ericsnowcurrently/backups-apiserver

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).