[SPARK-12153][SPARK-7617][MLLIB] add support of arbitrary length sentence and...

[SPARK-12153][SPARK-7617][MLLIB] add support of arbitrary length sentence and other tuning for Word2Vec

add support of arbitrary length sentence by using the nature representation of sentences in the input.

add new similarity functions and add normalization option for distances in synonym finding
add new accessor for internal structure(the vocabulary and wordindex) for convenience

need instructions about how to set value for the Since annotation for newly added public functions. 1.5.3?

jira link: https://issues.apache.org/jira/browse/SPARK-12153

Author: Yong Gang Cao <ygcao@amazon.com>
Author: Yong-Gang Cao <ygcao@users.noreply.github.com>

Closes #10152 from ygcao/improvementForSentenceBoundary.