Fixed prematching bug

When making PredicateArgumentIndexers, indexicals were being treated as
constants but should be treated as variables.  Fixed.
Improved stack traces for InferenceStepsExceededException
Fixed bug in "you" pronoun

Had had the speaker as its LF, rather than the addressee.
Pronouns override other realizations of NPs.
Added bulletproofing to Prolog calls
Added generating_nl/0
Implemented placeholder player dialog