Add option to justify text on wysiwyg editor.

#4384 

Before justification:
![selection_027](https://f.cloud.github.com/assets/4497748/1481545/85e4b566-46cb-11e3-8497-c3c0742ce4bb.png)

After justification:
![selection_028](https://f.cloud.github.com/assets/4497748/1481547/8e7b1580-46cb-11e3-8799-e253f403da87.png)