Improved handling of unknown terminal types:
- Eliminated the need to worry about unknown terminal types by replacing infrequent terminal strings with a specia symbol during training. The default value of cutoff is 1.
- Estimating distributions over this special "rare"/"unknown" terminal type actually improves performance slightly. Currently we have (pcfg-cky without gold tags) 74.6 F1 score on dev393 at 0.29 SPS.