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.

The code in this `for` loop would be more expressive and readable with tuple assignment. Something like:

    for donor, donations in donor_dict.iteritems():
        ...
        donor, sum(donations.values()), len(donations)
        ...