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
I didn't try running it (I'll trust you did ;-) ) -- but looks great!