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.

I strongly recommend that you allow a user to enter shorter phrases when interacting with your programs. Consider assigning a number to each option, accepting the first word, or the first letter of a phrase. 

Those options can then be placed in a set:

    if main_input.lower() in {u"1", u"s", u"send", u"send a thank you"}:
        ...

Alternatively, you can remove the series of `if` statements by placing all valid options in a dictionary and map them to the appropriate function:

    options = {
        u"1": send_a_thank_you_menu,
        ...
        u"2": show_list(),
        ...
        }

        # The following line isn't strictly necessary. 
        main_input = main_menu()
        # The following line should be wrapped in a try/except
        # that will prompt the user for valid input if there's a
        # KeyError.
        options[main_input]()