[SPARK-6095] [MLLIB] Support model save/load in Python's linear models

For Python's linear models, weights and intercept are stored in Python.
This PR implements Python's linear models sava/load functions which do the same thing as scala.
It can also make model import/export cross languages.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #5016 from yanboliang/spark-6095 and squashes the following commits:

d9bb824 [Yanbo Liang] fix python style
b3813ca [Yanbo Liang] linear model save/load for Python reuse the Scala implementation