Nounification

* Modify a bit the data structure. New object: Word.
* Perform nounification (e.g. die -> death).

Use this [StackOverflow post](http://stackoverflow.com/a/16752477/4110059) in this [function](https://github.com/ProjetPP/PPP-NLP-classical/blob/ba3ffacef40b6fc2a86ad03c642762cdad687abe/ppp_nlp_classical/preprocessing.py#L118)
From what I understood [here](http://blog.stackoverflow.com/2009/06/attribution-required/), this is allowed. If someone can confirm...
From what I understand, this code does the trick and is more readable:

```
pred = lambda l: l.synset().name().split('.')[1] == 'n'
for drf in derivationally_related_forms:
    related_noun_lemmas_lists.append(filter(pred, drf[1])))
related_noun_lemmas = list(itertools.chain(*related_noun_lemmas_lists))
```
(yes, I actually spent 15 minutes understanding this one line)