Add heat maps (as optional rendering?)

[stringfellow]

As per @jimr's suggestion in #5: http://www.patrick-wied.at/static/heatmapjs/example-heatmap-leaflet.html

[jimr]

I think ultimately you'll want to be able to switch between markers, clusters, and heat map (possibly not offering markers if there are more than n being displayed). I think clusters makes sense as the default...

[stringfellow]

Yep, sounds sensible :-)

[jimr]

Might need to do some work to make the heat map code work with GeoJSON but I'm sure we can figure it out.