[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
It may be better if we use `fit[S <: Iterable[String]](dataset: RDD[S])`. So users can use their own tokenizer. Example:

https://github.com/apache/spark/blob/master/mllib/src/main/scala/org/apache/spark/mllib/feature/HashingTF.scala#L69