Added feature transformer subsection to spark.ml guide, with HashingTF and Tokenizer.  Added JavaHashingTFSuite to test Java examples in new guide.

I've run Scala, Python examples in the Spark/PySpark shells.  I ran the Java examples via the test suite (with small modifications for printing).

CC: mengxr

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

Closes #6093 from jkbradley/hashingtf-guide and squashes the following commits:

d5d213f [Joseph K. Bradley] small fix
dd6e91a [Joseph K. Bradley] fixes from code review of user guide
33c3ff9 [Joseph K. Bradley] small fix
bc6058c [Joseph K. Bradley] fix link
361a174 [Joseph K. Bradley] Added subsection for feature transformers to spark.ml guide, with HashingTF and Tokenizer.  Added JavaHashingTFSuite to test Java examples in new guide