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