Start keeping a change log.

Getting the ball rolling with the latest release. We can work backwards later but hopefully
people can start updating the change log when making a release from now on.

Looks like this rendered: 

![image](https://cloud.githubusercontent.com/assets/65950/5422923/8094e810-826b-11e4-9a8d-db5a53836b37.png)

Fixes #208.