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.  
I don't like the name of this method/observer -- I don't think `document_publication` belongs in the name of this observer. It's an observer that, when called, sends the document to govdelivery as a new email. The fact it's being used only from the publication observer array is incidental (none of the other observers specify what "stage" the document has to be in, or that it's a document that's being sent).

I'd prefer something like `email_alert_exporter` to maintain consistency with the other exporters.