Add utilities for some common mapbox paint operations.

[chrisdevereux]

We provide the ability to customize our mapbox widgets with custom themes and styles, but actually using this requires people to use the relatively low-level mapbox style DSL.

Add some python helpers to produce the mapbox json for common requirements, such as:

• Showing markers for datapoints within the current view.
• Rendering some html into a popover for a given datapoint (possibly by fetching html over the wire).
• Shading a geographical area with a data-driven color. Support user interactions on painted areas.
• Rendering points onto the map via geojson. Support clustering and fetch-on-demand.