Daily digest email

Hey!

Not done with this, but want to get eyes on early. For: #1060

Daily email that sends user their unread notifications & a few pending submissions to nitpick.

Screenshot: https://www.dropbox.com/s/9cxd1regl4zszsd/MailCatcher_20140105_215935.jpg

notes:
+ only sends if user has notifications or pending submissions
+ didn't write a rake task yet to send it out
+ added ability to ```Email``` to send in html format.

Also, I just saw the changes to notification... so I'll have to do some changes to update to latest.