Extract ResponderProxy from persistence aspect

Clean up and generalise adding callbacks to the responder object
Update local Postgres config

There is a vagrant user created for Postgres on the development VM. The
email_alert_api user was only ever manually created.

This supercedes eb53232 which was accidentally blatted in a rebase by
3793cdc.
Remove obsolete migrate script

We have a db:migrate rake task so this isn't needed any more.
Merge pull request #29 from alphagov/add-db-migrate-rake-task

Add db migrate rake task
Merge pull request #27 from alphagov/cleanup-callbacks

Extract ResponderProxy from persistence aspect