Use the underlying tract's data for the bubble color (double-encode)

Modifies an existing function which determines tract style to accept a different `baseStyle` to extend, then uses that function to set the color of each bubble.

Right now, using 40% for opacity of the census tracts and 90% for the bubbles.

![screen shot 2014-08-05 at 1 22 45 pm](https://cloud.githubusercontent.com/assets/326918/3815661/53962dce-1cc7-11e4-9ce1-d81bcce04f01.png)

@ohsk: Salright?