Topic Jumper

Adds a widget to the progress bar to jump to specific posts.

Live demo: https://forum.riking.org/t/how-not-to-write-a-technical-book/571/44

The styling could really be improved @awesomerobot

![image](https://cloud.githubusercontent.com/assets/627891/3086559/a3b4eff8-e53a-11e3-8cdc-1780dc41e8b9.png)