Change return values of mnl_estimate

Now returns a dictionary of log likelihood values
(keys: 'null', 'convergence', 'ratio') and a DataFrame with
columns 'Coefficient', 'Std. Error', and 'T-Score'. The
DataFrame is not given any special index in mnl_estimate,
but when it is used with MNLLocationChoiceModel it gets an
index from the design matrix column names return by patsy,
which are derived from the model patsy expression.
Add function for converting Series to yaml-able

Converts index and values separately to native types and
zips them together into a dict.
Function to converting DataFrames to YAML-safe dicts.
Removing some unused fields from YAML ordering list.
Function for converting YAML from different types of sources.
Update YAML IO for MNLLocationChoiceModel

Use new YAML loading utility function and save model fit scores
and parameters.
Update RegressionModel to use new YAML IO utilities.
Have ordered_yaml put an extra newline between top level keys.
Merge pull request #22 from synthicity/mnl-estimate

Improved mnl_estimate return values and YAML utilities