Initial Sphinx organization.
Barebones pages for urbansim.models modules.
Barebones pages for some urbansim.utils modules.
Fix some Sphinx build errors/warnings.
Barebones pages for urbansim.urbanchoice mnl and interaction modules.

Very little to see there at the moment because most of the functions
do not have docstrings.
Add short module-level docstrings in several modules.
Put API docs under and "API" heading.
Have undocumented functions included in docs (where those exist).
Use the ReadTheDocs Sphinx theme.
Merge pull request #30 from synthicity/sphinx-docs

Sphinx docs