Renaming cli, clj, cl to i_weight, j_weight, weight_sum. About factoring it out as a function, I know it would be more extensible, I have opened a discussion in [nltk-dev](https://groups.google.com/forum/?fromgroups=#!forum/nltk-dev) regarding what should be added in general to hierarchical clustering in NLTK. Since the name of the class is GAAClusterer (Group Average Agglomerative) I don't really see the reason to factor it out as a function.

I do see the reason to factor out self._average_similarity though. Maybe we should discuss it in nltk-dev.