Remove unused import and variable
Add notes about stemming performance
Add stemming & scikit TF*IDF vectors to classifier

* Use snowball stemming
* Use TF*IDF vectors from scikit:
  - In order to combine stemming (nltk) with TF*IDF vectors, I created
    a custom tokenizer() function (inside the code of the function
    get_samples_predictions())
Merge branch 'master' of github.com:sortizm/TextMiningPrototypes