TensorFlow compatibility problems

[TITAN-PyCompat]

Hi @twiceyuan @whyscience @CreatCodeBuild ,

I observed some TensorFlow api usage in you code(Season2/1 Word2Vec/word2vec_tf.py) that might lead to serious compatibility problems(like crash).

For tf.global_variables_initializer appears in tf v0.12.0-rc0 and this will induced a crash if used in earlier version.

These problems can be fixed in pr #37

I judged the version and made some patches to make it compatible on more versions.
Look forward to your reply!
Thanks!

[CreatCodeBuild]

@TITAN-PyCompat Thanks and merged!