geojson-vt integration

[wilg]

A couple gotchas I could use some help with:

• A setTimeout is required because the map layer seems to require the tile fetch to be asynchronous lest I get a typeof this._tiles[canvasId] === 'undefined' when zooming and panning around.
• Does more manipulation of data than is necessary to get the vector tile to load with new VectorTile()

closes #37

[kentr]

@wilg For the async problem, did you look at mimicking the XMLHttpRequest load event system?

[wilg]

@kentr Not sure exactly what you're envisioning?

[kentr]

@wilg Nevermind.

I was thinking maybe the async problem could be solved by a something like a promise or subscribe / emit, but I dug into it and saw that there's more going on.