Added VSM code to generate a global tf-idf model. Not yet tested on a list of featureSet objects, but works on a list of texts.
Tested VSM on a real featureList; it works
Ported entropy calculation to use wcMatrix from provided VSM.
Ported calc_parent_overlap to vsm, using TFIDF cosine distance.
Updated demo to latest version, demonstrating use of VSM class.