Move backupsMetaC into backups.go.
Move state/backups.go to state/backups/storage.go.
Use the common implementation for getting a new State.
Add a note about dropping the corresponding function in state.
Use *state.State rather than the temp surrogate interfaces.
Fix a couple storage names.
Fix a missed change.
Merge pull request #1182 from ericsnowcurrently/backups-storage-go

Move state/backups.go to the state/backups/storage.go.

This eliminates the state/backups dependency in state.  See https://bugs.launchpad.net/juju-core/+bug/1392520.

(Review request: http://reviews.vapour.ws/r/493/)