Make footer small

This removes most of the content from the footer and replaces it with a focused list of links outside the site.

New:

![footer-small](https://cloud.githubusercontent.com/assets/6269/2625341/09a5f30c-bd90-11e3-83c7-b090ea67c0a6.png)

Compare with the original:

![footer-big](https://cloud.githubusercontent.com/assets/6269/2625338/01d74cac-bd90-11e3-89bb-3ce9cf0a6bc2.png)

I’m not sure about what links should be in there. The GitHub repo isn’t in there at the moment, but probably should be.

The age of big footers is over!

[Also on staging](http://staging.nanoc.ws/_branches/small-footer/).