PR with some minor edits

[rogerhoward]

Great job in general - nicely commented, your style looks good. Without unnecessary complicating the code (eg., modularizing it, which at this stage wouldn't improve anything), I had only a few suggestions:

1. Added a shebang to the top of the file, and chmod +x the file, to make it executable without explicitly calling the python interpreter
2. Changed your string formatting style to the modern style for better forwards compatibility
3. Added a requirements.txt file, which is the convention for declaring (and installing) dependencies such as NLTK

A couple other notes:

1. I would never recommend using pip install without using a Python virtualenv, but I didn't want to bikeshed too much
2. You could handle the setup steps - particularly the nltk.download - automatically, otherwise (I know from experience) this will be skipped and confusing may occur.

Hope this helps!