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.

`dict.setdefault()` returns a value. As such, you could combine line 66 with line 67:

    donor_dict.setdefault(donor, {})[date_today] = reply