Create util file

Adds util.py, a module to hold convenient utility functions related
to the task of classification.
Documentation for sentiment classification

Adds some documentation describing key sentiment methods in classify.py.
Build representative samples for train, dev, test

emoticon.py now also constructs randomized samples of training,
dev, and test datasets, using a 60/20/20 split among them.

Cursory tests show that the classifier has a large false
positive rate.