add commit message as a file to the changes browser

- similar to gerrit's web UI the commit message is now displayed as
  a file in the changes browser view
- comments can be viewed and added just like with other files
- resolves #87

![screen shot 2014-06-15 at 05 41 14 pm](https://cloud.githubusercontent.com/assets/3630295/3282440/0e56cc64-f4ef-11e3-892e-3b06cb1e7837.png)