[SPARK-7577] [ML] [DOC] add bucketizer doc

CC jkbradley

Author: Xusen Yin <yinxusen@gmail.com>

Closes #6451 from yinxusen/SPARK-7577 and squashes the following commits:

e2dc32e [Xusen Yin] rename colums
e350e49 [Xusen Yin] add all demos
006ddf1 [Xusen Yin] add java test
3238481 [Xusen Yin] add bucketizer