[SPARK-7104] [MLLIB] Support model save/load in Python's Word2Vec

Author: Yu ISHIKAWA <yuu.ishikawa@gmail.com>

Closes #6821 from yu-iskw/SPARK-7104 and squashes the following commits:

975136b [Yu ISHIKAWA] Organize import
0ef58b6 [Yu ISHIKAWA] Use rmtree, instead of removedirs
cb21653 [Yu ISHIKAWA] Add an explicit type for `Word2VecModelWrapper.save`
1d468ef [Yu ISHIKAWA] [SPARK-7104][MLlib] Support model save/load in Python's Word2Vec