added ggdendrotracks function in tracks.R

[chr1swallace]

I like in the pheatmap library being able to add annotation tracks to heatmaps and wanted to be able to do the same with a dendrogram. I added a function ggdendrotracks to do this in R/tracks.R. You can see the output in the vignette. It has added a couple of dependencies, on the packages scale and reshape.

[andrie]

Thank you. I shall take a look at this soon.