Display changelog in follower email

:fork_and_knife: Display truncated changelog and link to full changelog in follower notification email.

![screen shot 2014-08-07 at 3 00 42 pm](https://cloud.githubusercontent.com/assets/316507/3847455/ff1cbc5e-1e65-11e4-8fa9-35d7edbf71c6.png)

Closes #664.