Inclusion of Hclust into Clustering.jl

Hi, 

Following the suggestions from the METATDATA maintainers, I made a patch for Clustering to absorb the package Hclust (Hierarchical clustering along the lines of R's hclust() and cutree()), sice this is also does a form of clustering.  This keeps the number of packages down.  

I did not further integrate the code, it seems to be reasonably independent of the existing code in Clustering. 

---david


Well, isn't that the beauty of the dispatch mechanism, that we can define functions with new type of arguments that still do a similar thing to existing ones?  The output interface still is the same: a single (maximum) value.  The input interface obviously is different. 

If maximum(::Array, ::Vector, ::Vector) would have a meaning in Base, I wouldn't want to overload this function.  

To be honest, I had assumed that this overloaded function would not be available outside the module, since maximum is not exported.  But I see that it is, anyway.