Phrase support based on word2phrase

First approach to learning phrases based on wordphrases. Please comment. Not test and proper documentation yet. But they will come. 
Well, in my little snippet I left the threshold as `if len(vocab) > max_vocab_size`. But doing it the "powerlaw" way (each `min_reduce` layer will probably be as large as everything that comes after it put together) is ok too -- I leave that to your judgement :)