Add basic HTML styling for GovDelivery emails

[Trello ticket](https://trello.com/c/CEw6S2Lm/430-format-of-emails-we-send-2)

This makes the e-mails palatable, if not completely finished. Further work on this should be done when we fix the template on GovDelivery.

# Before
![before image](https://www.dropbox.com/s/kmaqcijztwugeu8/Screenshot%202014-11-20%2011.46.48.png?dl=1)

# After
![after image](https://www.dropbox.com/s/q1r8d8lafcawb68/Screenshot%202014-11-20%2011.45.24.png?dl=1)