Fullscreen editor

You can now make the editor span the full width of the window and additionally I replaced all background images by `border-image` properties to allow a higher quality look of editor and notifications. `background-image` is still used as a backup solution for browsers that do not support `border-image`.

![image](https://cloud.githubusercontent.com/assets/1505101/3529623/8d3de3e6-079b-11e4-815a-ec81314a8778.png)