Cluster close markers together on the map

Here's a proof-of-concept for map marker clustering. i wanted to share this to expand the discussion started in jlord/sheetsee.js#10.

i'm still having an issue where my first click on the first marker shown gives the text `Undefined` for the popup. i think there's some mapbox.js interaction i'm missing there.

i also increased the zoom level when clicking on a location in the table because the previous zoom value wasn't up close enough to distinguish many of the markers after clustering.