[SPARK-2429] [MLlib] Hierarchical Implementation of KMeans

I want to add a divisive hierarchical clustering algorithm implementation to MLlib. I don't support distance metrics other Euclidean distance metric yet. It would be nice to support it at other issue.
Could you review it?

Thanks!

I added `toString` method to `HierarchicalClustering`.
https://github.com/yu-iskw/spark/commit/2898c3fb0b99697f5600f584f7051b12830a75e0