Initial support for scikit-learn

[hollinwilkins]

Build out a Python module to serialize Scikit-learn + Pandas transformer pipelines to MLeap. We do not need to support deserialization to start.

1. Support Bundle.ML JSON format
2. Support several feature transformers, regression algorithms and classifiers
3. Make sure decision tree serialization is working properly
4. Publish module to PIP when we release MLeap 0.5.0
5. Add documentation for Python SK Learn integration
6. Create some notebooks showing SK learn