fix cosine_distance for KMeansClusterer
fix cosine_distance for KMeansClusterer

see issue 709 http://code.google.com/p/nltk/issues/detail?id=709
in short, cosine_distance is implemented as a similarity metric,
while KMeansClusterer expects a distance metric.

nltk/nltk,nltk/nltk,nltk/nltk