* Code from tweet_model.py now divided in two source files: tweet.py
  (containing the Tweet class) and utils.py (which contains the rest of the
  utilities to instantiate Tweet objects from CSVs, return a mini-dict with
  partial information of a Tweet, etc.)
* Added two new fields to the Tweet class: "polarity" and "trtext", which are
  not part of the original Tweeter tweets, but needed in other project.
* Added setter methods for "polarity" and "trtext", which will be probably now
  set at instantiation time but afterwards