Sphinx docs

Basic Sphinx docs for urbansim. Mostly this is to get the automatically generated API docs from the source code. Because of our dependency on a number of libraries that require compiling C and Fortran source we can't easily use Read The Docs, though see [this question in their FAQ](http://read-the-docs.readthedocs.org/en/latest/faq.html#i-get-import-errors-on-libraries-that-depend-on-c-modules).

For now I've put the docs on GitHub Pages at http://synthicity.github.io/urbansim/.