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