MNL Tests

I generated test data and results using R's mlogit package and used those to validate the numbers from our MNL package for both estimation (comparing coefficients) and simulation (comparing probabilities). The only change I had to make to mnl.py to get a good match (to one part in 10000) was to [lower the threshold](https://github.com/synthicity/urbansim/commit/e4ed2a41de5a6e0dc3ce7b482bed21d41b03d33c#diff-ece7c6f0bcdf6ac6893a982655d99bceR203) that terminates the SciPy optimization.