add a random hmm

this just provides a way of generating a random hmm, given a
`sentence_iterator`. it looks like it takes under a second to generate a
full random hmm from the entire wsj data set. evaluating it though takes
much longer.

@calvinmm @abhibhad if you guys can think of ways we can easily
make this less-meaningless, please let me know. note that the results
from the forward algorithm will be far greater than one because we're
not using a real probability distribution.