Finish cleanup of unigram/ngram memoization unification
Change --rerank-classifier default to avg-perceptron
Remove unused blank_memoized_string
Search-replace: item/etc. -> gram
Add/use forwarders in LangModel for .model funs/vars
Fix bug in K-d tree interpolation due to separate grid/rerank lm's
Generalize WordCellDist to GramCellDist (but GenerateKML not yet fixed)