initial (hacky) support for Mapbox vector tiles
proof of concept, needs to be extracted to properly support multiple formats/sources
merge overzoom
switch to mapzen fork of vector-tile-js (has geojson methods)
start extracting format/source-specific loading to new TileSource object
continue extracting and generalizing tile sources/formats
allow easier switching between mapzen geojson and mapbox mvt
tile source presets can be set in URL
update readme with tile format and example URLs