[SPARK-8456] [ML] Ngram featurizer python

Python API for N-gram feature transformer

Author: Feynman Liang <fliang@databricks.com>

Closes #6960 from feynmanliang/ngram-featurizer-python and squashes the following commits:

f9e37c9 [Feynman Liang] Remove debugging code
4dd81f4 [Feynman Liang] Fix typo and doctest
06c79ac [Feynman Liang] Style guide
26c1175 [Feynman Liang] Add python NGram API