Overhauling real/fake classifier

Removed POS features since POS tags on randomly generated tweets are
basically garbage.

Added support for passing in a list of 1 or more types of featurizers on
the command line to support mix-and-matching for experimentation.
Currently, the supported featurizers are unigram/bigram/trigram.
Add missing semicolon to tshrdlu script
Support gappy bigram features
Add "standard" gappy bigram support

Consider the sample tokens "the cat ate fish"

While the gappy bigrams implemented previously accounted for the size of
the gap by generating features like:
  "the+_+ate"
  "the+_+_+fish"

The "standard" implementation does not account for the size of the gap.
For example, it will create features like:
  "the+___+ate"
  "the+___+fish"