[#96] Notifying @mentions via email

* Create post_notification table to track who has already been emailed per post
* Create post observer and mailer
* Fix case sensitivity issues
* Ensure posts in private topics don't notify people it shouldn't
* Get tests passing on full rake
* Removing hard-coded references to thredded.com wrt incoming/outgoing emails
* Clean up markup in setup views
* Add incoming_email_host setting for site(s)
* Add extra header to post mailer for tracking outgoing email analytics
[JRO+SB] User can set preferences per messageboard

Users should be able to opt out of the notifications if they don't want.
Editing your account and preferences will allow you to do this.

* It saves your @ preferences
* Update some gems
* Rename notifiable_members to at_notifiable_members (more clear)
* Add constraint on preferences table to not allow nulls on messageboard_id