Implement Email Delivery

Implement email sending in CoInvitesController::send() (look for "XXX email the invitation, don't just render it") using CakeEmail (http://book.cakephp.org/2.0/en/core-utility-libraries/email.html).

This will require customizing the email configuration file, which should be done via the setup shell.

View/CoInvites/send.ctp might need updating or to go away completely.

Some or all of CoInvitesController::send() might be better off being refactored into the model.