Clean up README and add Git best practices

![screen shot 2014-07-30 at 11 08 53](https://cloud.githubusercontent.com/assets/72141/3747677/90b4db1a-17d1-11e4-8e6f-22f3f1646a5b.png)

- Fix some spelling issues
- Clean up trailing whitespace
- Clarify rebasing feature branches onto master
- Add commit message best practices