Show topics with exercises as available

New homepage view, without videos downloaded:
![image](https://f.cloud.github.com/assets/4072455/2358957/3323f340-a612-11e3-8a7d-cf695c1e789b.png)

A bit more inviting, if not mottled.  Also added a couple of basic tests.

![image](https://f.cloud.github.com/assets/4072455/2358973/53902ea0-a612-11e3-8f6b-daf5e0a516f2.png)