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)

What does this form look like without `.issuable-description` class? If the styling gained here is necessary I think it may make sense to create a new class with the same style or change the class name. The wiki form is not an 'issuable' form.