This PR allow Python users to set params in constructors and in setParams, where we use decorator `keyword_only` to force keyword arguments. The trade-off is discussed in the design doc of SPARK-4586.

Generated doc:
![screen shot 2015-02-12 at 3 06 58 am](https://cloud.githubusercontent.com/assets/829644/6166491/9cfcd06a-b265-11e4-99ea-473d866634fc.png)

CC: davies rxin

Author: Xiangrui Meng <meng@databricks.com>

Closes #4564 from mengxr/py-pipeline-kw and squashes the following commits:

fedf720 [Xiangrui Meng] use toDF
d565f2c [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into py-pipeline-kw
cbc15d3 [Xiangrui Meng] fix style
5032097 [Xiangrui Meng] update pipeline signature
950774e [Xiangrui Meng] simplify keyword_only and update constructor/setParams signatures
fdde5fc [Xiangrui Meng] fix style
c9384b8 [Xiangrui Meng] fix sphinx doc
8e59180 [Xiangrui Meng] add setParams and make constructors take params, where we force keyword args