editing preview

This PR allow to see markdown preview or diff during editing via web editor. After making changes you can click to diff or preview button and you will see:
![preview1](https://f.cloud.github.com/assets/1488195/1908245/71fef18e-7cee-11e3-9aea-ae03f45f8ace.jpg)
![preview2](https://f.cloud.github.com/assets/1488195/1908254/c28cb136-7cee-11e3-8b2a-d3aa5cb453b4.jpg)