fixed opener button & description links

The opener button:
![image](https://cloud.githubusercontent.com/assets/1074881/4433279/2df3e4f0-46cd-11e4-95d3-dd4765694c48.png)
has been broken ever since https://github.com/stackgl/shader-school/pull/98.

This PR fixes it and also removes the hardcoded numeric prefixes from each exercise's readmes.