[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
`_model` is not necessary. Shall we change the constructor to

~~~
class Word2VecModel private (private val model:RDD[(String, Array[Double])]) {
~~~

`Serializable` may be unnecessary.