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. 
True. If memory is a concern maybe splitting it to three separate lines is better: `vocab = vocab.items(); vocab.sort(key=...); vocab = dict(vocab[:self.max_vocab_size * 0.7])`, to avoid the temporal copies.

Not sure if it's not premature optimization though, at this point. Let's check against a large corpus to see how it fares.