Change CSS of headings to distinguish them better

I found it very hard to tell different heading levels apart, especially `h2`s and `h3`s. I have changed their sizes and added a subtle line below `h2`s so that they are more distinguishable, but still readable and not too small.

### Before

![about:jisp before heading style changes](https://cloud.githubusercontent.com/assets/79168/4181208/1417870a-3712-11e4-94f1-ddce2fd4193e.png)

### After

![about:jisp after heading style changes](https://cloud.githubusercontent.com/assets/79168/4181209/1bf899dc-3712-11e4-9729-fa6508136d9a.png)