Tidy up
New Conversation Notifications (via MessageMailer)

This commit effectively adds New Conversation Notifications by enhancing the existing message notification system.

Conversation#mailing_list is added to return the list of people to send emails to regarding that Conversation. This list contains the participants (people who have actually authored messages) and if assigned the agent. If unassigned the entire team (Account.people) are added.
Merge pull request #104 from austinylin/239-conversation-mailer

WIP 239 Conversation Notifications