Add Python bindings to StreamingLogisticRegressionwithSGD.

No Java wrappers are needed as models are updated directly using train.

Author: MechCoder <manojkumarsivaraj334@gmail.com>

Closes #6849 from MechCoder/spark-3258 and squashes the following commits:

b4376a5 [MechCoder] minor
d7e5fc1 [MechCoder] Refactor into StreamingLinearAlgorithm Better docs
9c09d4e [MechCoder] [SPARK-7633] Python bindings for StreamingLogisticRegressionwithSGD