Add anchors to markdown rendered headers

Fixes ACCEPTING MERGE/PULL REQUESTS at http://feedback.gitlab.com/forums/176466-general/suggestions/5158863-markdown-headers-should-have-proper-id-s-and-popup

Screenshot:

![screenshot from 2014-01-31 18 05 08](https://f.cloud.github.com/assets/1429315/2052664/516b1d26-8a9a-11e3-897a-10023f3143c3.png)

I'll finish implementation and write the tests once we agree on how the UI should look exactly (lets continue discussion there).