Features: remove categories and untrain tokens

I'm on a project where I need the ability to untrain tokens from a category on demand.

These commits add the two features: **deleting a category from the classifier data** and **untraining already trained tokens**.

Notes concerning the untraining of tokens:
* It decrement the number of examples for the category.
  If the being-untrained category has no more examples, it is removed from the category list.
* It untrain already trained tokens, non existing tokens are not considered.