Add margin below message headers.

Adds a 10px margin below the line that separates the message headers and the message. I was working on a responsive email that has no margins (email clients always create their own margins) and my content was right against the border. Others may have the same display problem.

![sample](https://f.cloud.github.com/assets/58460/944292/9b443e74-02a4-11e3-9ad1-1f3cfc5a1bb2.png)