add a pref for the editor font size

fixes #663 - add the ability to adjust the font size in the editor

![screen shot 2014-05-08 at 10 43 52 am](https://cloud.githubusercontent.com/assets/1269969/2919175/6c41b8e6-d6d8-11e3-9336-cdf59bd4a1c9.png)

@dinhviethoa, @ussuri