Phrase support baed on word2phrase

First approach to learning phrases based on wordphrases. Please comment. Not test and proper documentation yet. But they will come. 
Assuming I didn't miss something critical :), I think this entire pruning construct could be replaced with a single line:

```python
if len(vocab) > self.max_vocab_size:
    vocab = dict(sorted(vocab, key=lambda item: -item[1])[:self.max_vocab_size * 0.7])
```