Added user guide sections with code examples.
Also added small Java unit tests to test Java example in guide.

CC: mengxr

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #6127 from jkbradley/feature-guide-2 and squashes the following commits:

cd47f4b [Joseph K. Bradley] Updated based on code review
f16bcec [Joseph K. Bradley] Fixed merge issues and update Python examples print calls for Python 3
0a862f9 [Joseph K. Bradley] Added Normalizer, StandardScaler to ml-features doc, plus small Java unit tests
a21c2d6 [Joseph K. Bradley] Updated ml-features.md with IDF