Small refactorings, some clean-up

I pulled out a few lines to a new function so an external function (from some Disco map/reduce for PyCon) can call in easily with words and counts and added a docstring. I also made a few pep8 changes and pushed a default to global level. Also fixed a bug where you get an error message if you have 0 words to draw, now you get a nice explanation. I'll use this for PyCon, feel free to reject if you don't like the tweaks.