[experiment, WIP] Handle laying out with larger font sizes

* Calculates FieldLabel size using getBBox instead of getComputedTextLength
* Take in an optional extra class for FieldLabel

![screen shot 2014-06-04 at 6 33 55 pm](https://cloud.githubusercontent.com/assets/206973/3192432/26a0987e-ece6-11e3-83e3-ca9212c63a6b.png)