Act as a Sphinx extension

[gfxstrand]

I'd like to be able to publish my personal favourite recipes on my website. Unfortunately, while the current sphinx output is fine for just turning a cookbook into HTML, it's not so great for embedding into an existing site. One idea I've had for doing this better is to make the PyCook package itself act as a Sphinx extension allowing you to read PyCook yaml files as if they were a regular part of the Sphinx document tree.