Map key

Add a key to the map that contains color-to-percentages as well as a selector for switching to a bucketed view. Bucketed view is implemented using hard-coded JS until the backend API can be integrated.

Also, don't draw tracts with zero population.
Is there any reason why ```minorityContinuousStyle``` and ```minorityBucketedStyle``` can't return the result of a call to a generic version of these two functions, passing in the percentage value? It seems like these two functions are mostly the same.