Extract ResponderProxy from persistence aspect

Clean up and generalise adding callbacks to the responder object
Merge pull request #27 from alphagov/cleanup-callbacks

Extract ResponderProxy from persistence aspect
Fix db:setup rake task

Connect to the `postgres` database instead of the `template1` database
to drop and create other databases. `template1` isn't really safe to
connect to, and if another user is connected to that database then the
command will fail.