Support FPGrowth algorithm in Python API.
Should we remove "Experimental" which were marked for FPGrowth and FPGrowthModel in Scala? jkbradley

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #5213 from yanboliang/spark-6264 and squashes the following commits:

ed62ead [Yanbo Liang] trigger jenkins
8ce0359 [Yanbo Liang] fix docstring style
544c725 [Yanbo Liang] address comments
a2d7cf7 [Yanbo Liang] add doc for FPGrowth.train()
dcf7d73 [Yanbo Liang] add python doc
b18fd07 [Yanbo Liang] trigger jenkins
2c951b8 [Yanbo Liang] fix typos
7f62c8f [Yanbo Liang] add fpm to __init__.py
b96206a [Yanbo Liang] Support FPGrowth algorithm in Python API