Add task to generate change log entries

This addresses #687 and #703.

Current output
```
$ rake changelog
Bug Fixes (2)
-------------

* Merge pull request #709 from shoes/transform_dispose [265600c]
* Only report stat/end relative to the parent if there is one [c82e763]

Contributors (13)
-----------------

Tobias Pfeiffer, Jason R. Clark, Eric Watson, KC Erb, Philipp Hansch, presidentJFK, Ivan Zarea, Elia Schito, KCErb, Brett Chalupa, Steve Klabnik, Eugene Smentyna, ashbb
```