Add popup option to geo_json()

[araichev]

This functionality would be useful, and adding it looks doable via Leaflet's onEachFeature: http://leafletjs.com/examples/geojson.html.

[wrobstory]

I thought this was going to be a quick fix. Turns out it's easy to add text popup content, but adding Vincent/Vega popups is a bit tricker because a number of methods and attributes aren't available on the geoJson object. I'll keep tinkering (and pull requests are welcome!)