urbansim.models package

Thought it was probably about time for this to rejoin the master branch since I've now implemented regression, transition, relocation, and MNL location choice models. I think, in theory, that someone working in Python could use these model classes to put together a complete multi-year simulation if they did the data wrangling and accounting on their own. Now I'll be moving on to augmenting these model classes with JSON config storage and higher level simulation and data management stuff.

You can see some basic usage of the regression and location choice models here: http://nbviewer.ipython.org/gist/anonymous/8b1894b665b4246e045d.

Most of the changes are new in `urbansim.models`. The only changes likely to affect existing code are in `urbanchoice/interaction.py`, `urbanchoice/mnl.py`, and `urbansim/dataset.py`. And I think most of those are trivial. The only real change there is to `urbanchoice.interaction.mnl_interaction_dataset` so that it doesn't return the string `'mnl'` as part of the return value. That probably breaks something downstream.