* 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

Added functions that allow to get dictionaries representing a subset of the Tweet object information

Since the last release the main improvements have been:

 * Raise a custom exception instead of sysexiting when confronted with a wrong
 CSV file.
 * Change prints for logger messages.
 * Modularize the "get_tweets_from_csv" dividing it into 3 functions to
 provide more isolated functions. Now this function makes use of
 "get_tweet_from_csv_line", which returns a single Tweet object when
 provided with two lists representing the contents of a CSV header and
 one of its entries. There is also a "raw" alternative of that
 function, which works with raw lines and converts them to lists to then
 call the "non-raw" version.

Pretty stable release with the basic intended functionality