Version 0.1beta3 Release

Final PR before publishing new version release to PyPI.

ModBetaGeoModel Bayesian implementation of Modified BG/NBD model added.
Added conditional_probability_of_being_alive_up_to_time to ParetoNBDFitter.
Removed CSV model persistence and added a validation check for loading models from JSON files.
Removed dill dependency for persisting fitters models.
Converted _check_inputs into a static method and added a validation check for 'frequency < T'.
Converted _dataframe_parser from a static method and removed unnecessary _check_inputs call.
Removed direct imports for BaseModel and PredictMixin.

btyd_models_uml

ColtAllen / btyd