[SPARK-5894] [ML] Add polynomial mapper

See [SPARK-5894](https://issues.apache.org/jira/browse/SPARK-5894).

Author: Xusen Yin <yinxusen@gmail.com>
Author: Xiangrui Meng <meng@databricks.com>

Closes #5245 from yinxusen/SPARK-5894 and squashes the following commits:

dc461a6 [Xusen Yin] merge polynomial expansion v2
6d0c3cc [Xusen Yin] Merge branch 'SPARK-5894' of https://github.com/mengxr/spark into mengxr-SPARK-5894
57bfdd5 [Xusen Yin] Merge branch 'master' into SPARK-5894
3d02a7d [Xusen Yin] Merge branch 'master' into SPARK-5894
a067da2 [Xiangrui Meng] a new approach for poly expansion
0789d81 [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into SPARK-5894
4e9aed0 [Xusen Yin] fix test suite
95d8fb9 [Xusen Yin] fix sparse vector indices
8d39674 [Xusen Yin] fix sparse vector expansion error
5998dd6 [Xusen Yin] fix dense vector fillin
fa3ade3 [Xusen Yin] change the functional code into imperative one to speedup
b70e7e1 [Xusen Yin] remove useless case class
6fa236f [Xusen Yin] fix vector slice error
daff601 [Xusen Yin] fix index error of sparse vector
6bd0a10 [Xusen Yin] merge repeated features
419f8a2 [Xusen Yin] need to merge same columns
4ebf34e [Xusen Yin] add test suite of polynomial expansion
372227c [Xusen Yin] add polynomial expansion