Changed method of handling removing an instance of an ngram from each reference that has it, cleaner now
and prevents a problem that arose when we ran out of reference_ngrams early by re-removing previous ones

Additionally, wrote the front end, somewhat. Now parses a file in argv[2] to get lines of the form:
mt-output ||| reference [| reference | reference...]
bleu now returns a list of four 2-tuples:
arr[i-1] = (i-gram relevant score, # of i-grams in output)