[SPARK-2852][MLLIB] API consistency for `mllib.feature`

This is part of SPARK-2828:

1. added a Java-friendly fit method to Word2Vec with tests
2. change DeveloperApi to Experimental for Normalizer & StandardScaler
3. change default feature dimension to 2^20 in HashingTF

Author: Xiangrui Meng <meng@databricks.com>

Closes #1807 from mengxr/feature-api-check and squashes the following commits:

773c1a9 [Xiangrui Meng] change default numFeatures to 2^20 in HashingTF change annotation from DeveloperApi to Experimental in Normalizer and StandardScaler
883e122 [Xiangrui Meng] add @Experimental to Word2VecModel add a Java-friendly method to Word2Vec.fit with tests
[PySpark] Add blanklines to Python docstrings so example code renders correctly

Author: RJ Nowling <rnowling@gmail.com>

Closes #1808 from rnowling/pyspark_docs and squashes the following commits:

c06d774 [RJ Nowling] Add blanklines to Python docstrings so example code renders correctly