Updated method: 'listing.illegal?' method to implement PhraseListing join table entry; by adding a unique entry for each instance of a phrase appearing in a listing description.
Added method: Listing.set_discriminatory, to provide a method to run all Listings through 'listing.illegal?' method.
Added method: Phrase.phrase_counts, which returns hash of key/value pairs (phrases and counts) derived from PhraseListing table.