Cluster together sections with same location+time in exam schedule

Fixes #45. I'm not 100% on if this is the best implementation, so happy to take suggestions. :-)

Before:
![image](https://f.cloud.github.com/assets/808263/2303288/0bbcff12-a1b3-11e3-88e4-880e8f16b57e.png)

After:
![image](https://f.cloud.github.com/assets/808263/2303287/ea7e90ea-a1b2-11e3-8d58-87999f0fd96d.png)