add option for font-size

add option for font-size

allows user to choose from the available options: xx-small, x-small, small, medium, large, x-large, xx-large.  defaults to medium which matches the current fault.  screenshot attached. as requested in #32.

![image](https://f.cloud.github.com/assets/166513/2387833/a486f2e0-a93b-11e3-847e-35a4a9854735.png)
![image](https://f.cloud.github.com/assets/166513/2387852/c6d7621c-a93b-11e3-8246-495dc32840c3.png)