[SPARK-9544] [MLLIB] add Python API for RFormula

Add Python API for RFormula. Similar to other feature transformers in Python. This is just a thin wrapper over the Scala implementation. ericl MechCoder

Author: Xiangrui Meng <meng@databricks.com>

Closes #7879 from mengxr/SPARK-9544 and squashes the following commits:

3d5ff03 [Xiangrui Meng] add an doctest for . and -
5e969a5 [Xiangrui Meng] fix pydoc
1cd41f8 [Xiangrui Meng] organize imports
3c18b10 [Xiangrui Meng] add Python API for RFormula