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