Support Pivotal Tracker's epic

Before:
![2014-06-19 20 19 43](https://cloud.githubusercontent.com/assets/290782/3326831/b5ec7b3e-f7a3-11e3-9e32-fc43d8ac42c9.png)

After:
![2014-06-19 20 19 54](https://cloud.githubusercontent.com/assets/290782/3326832/b5ed361e-f7a3-11e3-91f4-015fe93f629f.png)
Can you introduce `Hook::PivotalTracker#epic?(story)` method?