Submitting Week 5 Homework

I had a little trouble figuring out where there were good places for list comprehensions in the mailroom.py app. I had the same problem with tests; I wasn't sure how to utilize them when I'm not specifically checking the return value of a function. 

Aside from that, everything should be just about done
this is simple enough to throw all in one test, but in general, you separate test for separate circumstances, so when one fails, you now exactly what went wrong.