Add .fit_parameters table to RegressionModel

This mirrors the same parameter on MNLLocationChoiceModel: after
the model is fit it is a DataFrame with columns Coefficient,
Std. Error, and T-Score, and the index is the model parameters.
The frame also has extra attributes: .rsquared and .rsquared_adj
with the same values as the model fit results from statsmodels.
Update YAML-ation of RegressionModel with fit_parameters

Instead of saving out just coefficients (params), save out a full
fit_parameters table as with MNLLocationChoiceModel. Also saves
new keys 'fit_rsquared' and 'fit_rsquared_adj' from the model fit.

_FakeRegressionResults is also updated to have the same attribute names
as statsmodels results and use the fit_parameters table as input
(as well as the rsquared values).
Add a .report_fit() method to RegressionModel.
Add option to use assert_allclose in DataFrame comparison utility.
Don't compare T-Score or Std. Error columns

In my toy fit they are going to by very small random numbers
so it's hard to write down an exact number for them that will
apply across systems.
Merge pull request #27 from synthicity/regression-stats

RegressionModel fit statistics reporting