Provide light clustering functionality for heatmaps

[cnluzon]

This would apply to:

• Summary heatmaps. Hierarchical clustering would be preferred as these heatmaps show small amount of rows. However ggplot does not provide dendrogram functionality, so in order to do this I should use R base functionality or just draw the dendrogram somewhere else.
• Classic heatmaps. K-means or something like this would be good.