First draft of new heading spacing/font size.

This is to address #3224.  In master, heading cells have too much top and bottom margin so notebooks take up too much vertical space.  I have made the following changes to fix that:

* Adjusted the font sizes to be 32px, 26px, 20px, 14px, 12px and 12px with underline.
* Removed the top/bottom margins.

Here is the new look:

![screen shot 2013-08-31 at 8 37 53 pm](https://f.cloud.github.com/assets/27600/1063420/7057b94c-12b8-11e3-887b-ed47378ed3ea.png)

And the old:

![screen shot 2013-08-31 at 8 42 41 pm](https://f.cloud.github.com/assets/27600/1063423/9e4b9012-12b8-11e3-8cb2-a4bb0abc1e67.png)

This needs more work:

- [ ] We currently set the main font size to 13px, but override that up to %110 to get 14px. This means that in nbconvert produced output, the font size will always inherit the outer container.  I think we should probably start to work in pixel font sizes inside the notebook div.
- [ ] If you put Markdown headings inside Markdown cells, the top/bottom margins are too small.  With our current css classes on cells, we don't distinguish between Markdown and heading cells, so I I am not how we want to fix this.