Major changes

Fix pomegranate version to v0.12.0. (#79)
Add serialization and deserialization support for all classifiers. (#80)
- HMM, HMMClassifier: Serialized in JSON format.
- KNNClassifier: Serialized in HDF5 format.
Finish preprocessing documentation and tests. (#81)
(Internal) Remove nested helper functions in KNNClassifier.predict(). (#84)
Add strict left-right HMM topology. (#85)
Note: This is the more traditional left-right HMM topology.
Implement GMM-HMMs in the GMMHMM class. (#87)
Implement custom, uniform and frequency-based HMM priors. (#88)
Implement distance-weighted DTW-kNN predictions. (#90)
Rename DTWKNN to KNNClassifer. (#91)

Minor changes