Speed improvement on GAAClusterer

New improved vectorized version of GAAClusterer that is orders of magnitude
faster than the previous one.
Updates to GAAClusterer so that the new cluster_vectorspace function
conforms to PEP 8.
Readability changes and comment additions to GAAClusterer

- Removed _average_similarity and imported cosine_distance
- Added _average_link to be clear about how the distance
  matrix is updated after merge
- Changed sim to dist since we are using distances instead
  of similarities now
Renaming _average_link to _merge_similarities
Merge pull request #418 from angeloskath/master

Speed Improvements in GAAClusterer