Add 3D height option to ChoroplethViz

[ryanbaumann]

Add support to extrude a Choropleth polygon layer. Expose the option as height* properties to the Choropleth Viz.

• height_stops
• height_property
• height_default

Example Viz: 3D Population Density - www.mapbox.com/bites/00273

[akacarlyann]

@ryanbaumann I've started on this. I basically lifted the scheme for calculating the weight stops for a heatmap or radius stops, but had to greatly exaggerate the height gradient for visibility on the map. I'll submit a PR (#90) for your review :)