[MLlib] [SPARK-2510]word2vec: Distributed Representation of Words

Vector representation of words. This is a pull request regarding SPARK-2510 at https://issues.apache.org/jira/browse/SPARK-2510
Btw, this is definitely an experimental feature. Please add `@Experimental` tag. Example:

https://github.com/apache/spark/blob/master/mllib/src/main/scala/org/apache/spark/mllib/linalg/distributed/RowMatrix.scala#L45