This is the sub-task of SPARK-6254.
Wrap missing method for `Word2Vec` and `Word2VecModel`.

Author: lewuathe <lewuathe@me.com>

Closes #5296 from Lewuathe/SPARK-6615 and squashes the following commits:

f14c304 [lewuathe] Reorder tests
1d326b9 [lewuathe] Merge master
e2bedfb [lewuathe] Modify test cases
afb866d [lewuathe] [SPARK-6615] Python API for Word2Vec