Changes to buttons and button groups

Adding in documentation for Even class on Button Groups and clarifying sections about Styling versus Colors.
![screen shot 2014-08-22 at 6 25 34 pm](https://cloud.githubusercontent.com/assets/7770587/4019307/72442b36-2a64-11e4-9d2c-054f9956e718.png)
![screen shot 2014-08-22 at 6 25 17 pm](https://cloud.githubusercontent.com/assets/7770587/4019308/724b7c2e-2a64-11e4-931f-67de4357c87a.png)