Allow going to previous or next set of exercises

Solves #305. Depends on #1239.


Thanks to `get_neighbor_nodes` by @bcipolli, this was easy to implement.


![2013-12-05-192457_1366x768_scrot](https://f.cloud.github.com/assets/191955/1689571/05ddb3d2-5e26-11e3-9c19-549b536f603a.png)