Refs #328: Use DB backend in httptester

- change httptester to use DB backend
- remove httptester's old backend
- update httptester doc
- add notes to 0.13.0 release notes
Refs #328: Cleanup use of backend name

- Use the backend name as the `name` in all the BackendMessages associated
  with the message tester, so we can view just those and easily clear them.
- Don't pass backend names in URLs and methods - just use a fixed backend
  name for HTTP tester, 'message_tester'
Refs #328: remove internal fields from message tester table display

@copelco ready to review again
Merge remote-tracking branch 'remotes/origin/feature/bulk-messaging-api' into feature/328-httptester-use-db-backend

Conflicts:
	docs/releases/0.13.0.rst
	rapidsms/backends/db/__init__.py
Rename database backend from `db` to `database`

This is because of a Django app name collision with the database router.
Use full name of database backend instead of DB backend.
Merge pull request #328 from rapidsms/feature/328-httptester-use-db-backend

Potentially switch httptester backend to new db backend