Widen emails for mobile (and desktop) viewing, fixes #132

//cc @jbarrett @sdougbrown @chrismorast 

I've attempted to make mobile emails work better. It seems our email template has a lot of (maybe too much?)  whitespace beside the content which looks okay on desktop, but awful on mobile -- specifically iOS. On android in Gmail and AquaMail the template is alright because the content is scaled and I'm able to zoom. I believe this isn't the case on iOS (don't have a phone to test with...).

I'm very new to HTML emails and from the little I've read it seems that CSS @media selectors are sometimes used but aren't very reliable as many popular email apps ignore them.

This fix basically widens the whole notification template.

Before:
![image](https://cloud.githubusercontent.com/assets/873785/3254198/e1877e0a-f1f0-11e3-9a21-20c1418eae4a.png)

After:
![image](https://cloud.githubusercontent.com/assets/873785/3254205/06a55c16-f1f1-11e3-8424-69b26d99e527.png)

If this is a step in the right direction, would either of you happen to know how I can test this?

Alternatively it seems that there might be some webkit specifc css settings we can use to prevent iOS from zooming the text -- that might be a quicker/easier fix. See http://stackoverflow.com/questions/11801685/issue-stopping-iphone-resizing-html-e-mails