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...
Could you avoid nesting list comprehensions?