[SPARK-5692] [MLlib] Word2Vec save/load

Word2Vec model now supports saving and loading.

a] The Metadata stored in JSON format consists of "version", "classname", "vectorSize" and "numWords"
b] The data stored in Parquet file format consists of an Array of rows with each row consisting of 2 columns, first being the word: String and the second, an Array of Floats.

Author: MechCoder <manojkumarsivaraj334@gmail.com>

Closes #5291 from MechCoder/spark-5692 and squashes the following commits:

1142f3a [MechCoder] Add numWords to metaData
bfe4c39 [MechCoder] [SPARK-5692] Word2Vec save/load