Add .form-group-sm/-lg

Fixes #13021.

![screen shot 2014-06-18 at 6 18 53 pm](https://cloud.githubusercontent.com/assets/98681/3322653/d25592b4-f74f-11e3-8e38-8f6c92df1758.png)

This is a bit janky because forms and what not, but it gets the job done. The exact technique for large groups vs small groups varies due to the sizing methods of our inputs. For that reason alone, I kind of question this addition. It feels like I'm patching something otherwise faulty, but I don't have a better idea yet.