Speed Improvements in GAAClusterer

I have added a similarity matrix to avoid recomputing the similarities. In addition the matrix update is done in a vectorized fashion to get the maximum NumPy has to offer.

The only thing that has been touched is the function cluster_vectorspace.

The result is more than 100 times faster clustering. It is an improvement (by better utilizing NumPy) from what is [mentioned to the nltk-dev group](https://groups.google.com/d/msg/nltk-dev/7D2ZTePSGFs/A83wLEm3AWIJ) as "First take on the problem".