Session04 Tasks

Task 1 - Dictionaries and Sets
Task 2 - Trigrams
Task 3 - Wrapper Function
Task 4 - Mailroom Madness
Checkout [`dict.setdefault()`](https://docs.python.org/2/library/stdtypes.html#dict.setdefault). You should be able to use to condense `add_donor()` and `update_donor()` into one function.