Use regression models for plotting etc

[erikbern]

This removes the old scipy.optimize-based models and switches everything to the tensorflow-based models I put together last night. Cleans up a lot of code and the new models should be faster and more robust.

Two things to fix shortly:

1. The plotting is a bit dumb – we're still fitting separate models for each group, so there's really no benefit from thinking about it as a regression model.
2. Compiling the model adds several seconds over overhead each time the model is fit. Should figure out a way to reuse the computational graph.

[coveralls]

Coverage increased (+4.3%) to 76.547% when pulling c152e19 on use-regression-models into b22b03c on master.