Implemented likelihood convergence criterion.

Everything with the models seems to be working and is up to speed with
the baselines in the assignment writeup and on Piazza. The constants
could be fiddled with, but let's worry about that when we do the
writeup.
Implemented a couple simple features that work well.

I implemented two features that together give an increase 0.36 in BLEU
score (averaged over 5 trials). One simple feature  just takes the
ratio of the lengths of the phrases on each side of a rule
(corresponding to the intuition that English is often more concise than
 French) and indicator features that fire when certain common English
prepositions are present in particular locations in a phrase
(corresponding to the intution that certain prepositions, e.g. on or in,
 translate quite directly whereas others, e.g. of or to, are often
 removed/changed in the English translation.