New functionality:

* Refactor `ImageReader` and `BinaryFileReader` to support streaming
  images, including a Python API.  Also improved performance of the
  readers.  Check the 302 notebook for usage example.

* Add `ClassBalancer` estimator for improving classification performance
  on highly imbalanced datasets.

* Create an infrastructure for automated fuzzing, serialization, and
  python wrapper tests.

* Added a `DropColumns` pipeline stage.

New notebooks:

* 305: A Flowers sample notebook demonstrating deep transfer learning
  with `ImageFeaturizer`.

Updates:

* Our main build is now based on Spark 2.2.

Improvements:

* Enable streaming through the `EnsembleByKey` transformer.

* ImageReader, HDFS issue, etc.