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. 
Initially I thought that the learned vocab could be somehow reused by the  word2vec / doc2vec module, so you don't have to learn twice the same vocabulary. However by doing the "pruning"  we are actually removing some unigrams that otherwise wouldn't. So I guess chaning this to a defaultdict makes sense.