Send email alerts on publish

This is the work to allow emails to be sent on publish to the govuk_delivery API. Currently the formatting of the email is relatively unimportant to us, however it should be simple enough to change the view templates later. It would be nice to be able to render view using Erb directly in order to remove the exporter class' dependency on ActionView. This is something have looked into however it is not as simple as i first assumed. For now lets use ActionView in order for us to be able to test email sending.  
Having spoken to @evilstreak I'm happy for this to continue -- we're aiming to bin the schema as much as possible -- it doesn't make sense to use it at the publication side, and this approach is consistent with the approach used elsewhere.