[SPARK-6267] [MLLIB] Python API for IsotonicRegression

https://issues.apache.org/jira/browse/SPARK-6267

Author: Yanbo Liang <ybliang8@gmail.com>
Author: Xiangrui Meng <meng@databricks.com>

Closes #5890 from yanboliang/spark-6267 and squashes the following commits:

f20541d [Yanbo Liang] Merge pull request #3 from mengxr/SPARK-6267
7f202f9 [Xiangrui Meng] use Vector to have the best Python 2&3 compatibility
4bccfee [Yanbo Liang] fix doctest
ec09412 [Yanbo Liang] fix typos
8214bbb [Yanbo Liang] fix code style
5c8ebe5 [Yanbo Liang] Python API for IsotonicRegression