[SPARK-4582][MLLIB] get raw vectors for further processing in Word2Vec

This is #3309 for the master branch.

e.g. clustering

Author: tkaessmann <tobias.kaessmanns24.com>

Closes #3309 from tkaessmann/branch-1.2 and squashes the following commits:

e3a3142 [tkaessmann] changes the comment for getVectors
58d3d83 [tkaessmann] removes sign from comment
a5be213 [tkaessmann] fixes getVectors to fit code guidelines
3782fa9 [tkaessmann] get raw vectors for further processing

Author: tkaessmann <tobias.kaessmann@s24.com>

Closes #3437 from mengxr/SPARK-4582 and squashes the following commits:

6c666b4 [tkaessmann] get raw vectors for further processing in Word2Vec

mllib/src/main/scala/org/apache/spark/mllib/feature/Word2Vec.scala

@@ -461,4 +461,11 @@ class Word2VecModel private[mllib] (
       .tail
       .toArray
   }
+  
+  /**
+   * Returns a map of words to their vector representations.
+   */
+  def getVectors: Map[String, Array[Float]] = {
+    model
+  }
 }