Added codemirror support to Waliki.

Ok, late but sure...
Codemirror support for markdown and rest (highlighting) and support for available codemirror themes.
All configurable with your `config.py` file
If you use `MARKUP='restructuredtext'` the codemirror mode (highlight) is setted automatically...
Then you can use `THEME='monokai'` or any theme available at `waliki/static/codemirror/theme` (by default I choose the the `elegant` theme if the user do not set any theme in your `config.py`).

Abrazo.

Damián