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.

Using `+` to combine two strings works, but it's generally better to use the `str.join()` method. "It's faster, uses less memory, and you'll see it everywhere anyway." -- [Jeff Knupp](http://jeffknupp.com/writing-idiomatic-python-ebook/)