Add some backups testing helpers.
Drop the "archivable" interface.
Clean up fake error handling.
Add some doc comments.
Move a private function below the public ones.
Clarify the content place-holders.
Merge pull request #869 from ericsnowcurrently/backups-fakes

Add some backups testing helpers.
Some more network management jobs added.
Merge branch 'master' of https://github.com/juju/juju into networker-mode-based-on-agent-api