Markdown preview in wiki

Added preview to wiki edit form, as it is done e.g. on issue edit form.

![screen shot 2014-12-24 at 4 53 07 pm](https://cloud.githubusercontent.com/assets/6962409/5549208/867027a6-8b8d-11e4-9605-701d11769948.png)

It is necessary for top margin. Added new wiki-content class.