[SPARK-8703] [ML] Add CountVectorizer as a ml transformer to convert document to words count vector

jira: https://issues.apache.org/jira/browse/SPARK-8703

Converts a text document to a sparse vector of token counts.

I can further add an estimator to extract vocabulary from corpus if that's appropriate.

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #7084 from hhbyyh/countVectorization and squashes the following commits:

5f3f655 [Yuhao Yang] text change
24728e4 [Yuhao Yang] style improvement
576728a [Yuhao Yang] rename to model and some fix
1deca28 [Yuhao Yang] Merge remote-tracking branch 'upstream/master' into countVectorization
99b0c14 [Yuhao Yang] undo extension from HashingTF
12c2dc8 [Yuhao Yang] Merge remote-tracking branch 'upstream/master' into countVectorization
7ee1c31 [Yuhao Yang] extends HashingTF
809fb59 [Yuhao Yang] minor fix for ut
7c61fb3 [Yuhao Yang] add countVectorizer