Use base model 'data' implementation for non Variable items.
Added Hierarchical Clustering widget.
Added hierarchical clustering icon.
Removed unused code.
Merge branch 'master' into hierarchical-clustering
Changed input/output type declarations to DistMatrix.
Moved hierarchical clustering functions...

... under Orange.clustering.hierarchical namespace.
Added basic unittests for hierarchical clustering.
Added GUI controls for cluster selection.
Remove unused code.
Merge branch 'hierarchical-clustering'