New styles & rebecca-and-greg(3) assignment.

* Homepage is now valid HTML5.
* Added CSS stylesheet for improved visual design.
* Grabs (47kB) Chaparral Pro from Typekit.
* Added Google Analytics.

* Added assignment for `rebecca-and-greg(3)`.

The new stylesheet should work for multi-level assignments as long as the whole `dl.assignment` is nested (within the `dd` of the previous level).