Added missing icons of the codemirror plugin

I just realized that the icons of the codemirror plugin are currently missing. This produces the following effect when trying to edit source code - rather unusable:

![codemirror-missing_icons](https://f.cloud.github.com/assets/1049882/1523447/48611690-4bb6-11e3-825a-86e82a945e02.png)

I grabbed the [icons](https://github.com/w8tcha/CKEditor-CodeMirror-Plugin/tree/master/codemirror/icons) from the [plugin repo](https://github.com/w8tcha/CKEditor-CodeMirror-Plugin/) and this fixes it.