Email template layout, header and Github icon adjustments.

##### ICLA/CCLA Mailer
![image](https://f.cloud.github.com/assets/438082/2222999/6433f806-9a6c-11e3-8946-47334e328a2d.png)

##### CCLA Contributor Invitation Mailer
![image](https://f.cloud.github.com/assets/438082/2223008/72983254-9a6c-11e3-952a-2af5853d0274.png)