Add editor's font size setting

This is referenced to #82, which is opened by me. :smile: 
If there is anything I can improve in this pull request, please tell me.

Larger font size on screenshots:
![screenshot from 2014-06-22 01 33 32](https://cloud.githubusercontent.com/assets/1662910/3350055/8a723f76-f98c-11e3-82d6-6c4a547166f9.png)
![screenshot from 2014-06-22 01 34 28](https://cloud.githubusercontent.com/assets/1662910/3350056/8e48d402-f98c-11e3-915c-2fc68e8d60ef.png)