Divided the capabilities view into groups

I needed to setup some new custom roles and selecting the correct capabilities was a bit cumbersome.
So I am really scratching my own itch here but maybe this is something that more people would love to have.

The checkbox before the group name checks or unchecks all the capabilities that belong to that group. 

![groups](https://f.cloud.github.com/assets/579455/1826294/dd62f160-71f3-11e3-9f9b-71bb914f4b56.png)

## TODO

- [ ] Support for other plugins like GravityForms