Fixes style distruptancies for sidenav on long paths and lists

This patches fixes some minor, yet annoying glitches when viewing long documentation pages:
- in the sidenav: for long paths the text would not break and the path was not readable
- in the sidenav: for long lists the affix sidenav was not distinguishable from the base sidenav and created visual clutter
![sidenav_affix](https://cloud.githubusercontent.com/assets/4658815/2688167/75792bac-c287-11e3-9a39-4443248dfe62.png)
![sidenav_long-paths](https://cloud.githubusercontent.com/assets/4658815/2688168/757ab3d2-c287-11e3-8a04-e1f560143c2f.png)