Session04

Add 'dict_set.py' file for dict lab assignment.
Add 'trigrams.py' file for trigrams assignment.
Add 'safe_input.py' for exceptions assignment.
Add 'mailroom.py' for mailroom madness assignment.

This works, but I find using `str.format()` a little more readable:

    today = date.today()
    date_today = u"{}-{}-{}".format(today.month, today.day, today.year)