Enhancements for single-linkage special case

[GoogleCodeExporter]

Purpose of code changes on this branch:

Update HAC to have the O(n^2) performance for single-linkage

When reviewing my code changes, please focus on:

- Comments.  Is it clear what's going on?

You might take a look at Stanford's IR book for the single-linkage algorithm 
(http://nlp.stanford.edu/IR-book/html/htmledition/time-complexity-of-hac-1.html)
  but it's a bit opaque at first.  I was hoping that this code might read more 
easily than theirs.

After the review, I'll merge this branch into:
/trunk

Original issue reported on code.google.com by [email protected] on 17 Mar 2011 at 12:25

[GoogleCodeExporter]

Have you committed this to the trunk?  If so, can you close out the issue and 
the code review?

Original comment by [email protected] on 23 Sep 2011 at 5:29