A simple wrapper to save/load `MatrixFactorizationModel` in Python. jkbradley

Author: Xiangrui Meng <meng@databricks.com>

Closes #4811 from mengxr/SPARK-5991 and squashes the following commits:

f135dac [Xiangrui Meng] update save doc
57e5200 [Xiangrui Meng] address comments
06140a4 [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into SPARK-5991
282ec8d [Xiangrui Meng] support save/load in PySpark's ALS