Color Gradient Heatmaps

[rsandor]

Use a spectrum from one color to another, or through multiple color stops, as opposed to using opacity to represent height in the heat map. Possible color combination [dark red, yellow].

[rsandor]

Implementation is a snap with the above implementation. It requires a few minor changes to the basic heatmap implementation and then a couple of overrides. The result looks very... pretty. Though I'd like to see it working on real data before moving forward.

[rsandor]

Also it would be neat to see multi-stop interpolation, say from blue -> red -> yellow, for instance.

[rsandor]

See notes in the close comment of #37.