Otherwise, users can only use `transform` on the models. brkyvz

Author: Xiangrui Meng <meng@databricks.com>

Closes #6156 from mengxr/SPARK-7647 and squashes the following commits:

1ae3d2d [Xiangrui Meng] add weights and intercept to LogisticRegression in Python
f49eb46 [Xiangrui Meng] add weights and intercept to LinearRegressionModel

Added LinearRegression Python API

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #6016 from brkyvz/ml-reg and squashes the following commits:

11c9ef9 [Burak Yavuz] address comments
1027a40 [Burak Yavuz] fix typo
4c699ad [Burak Yavuz] added tree regressor api
8afead2 [Burak Yavuz] made mixin for DT
fa51c74 [Burak Yavuz] save additions
0640d48 [Burak Yavuz] added ml.regression
82aac48 [Burak Yavuz] added linear regression