Add new GeoPack format for VecTiles

GeoPack is a small adjustment to [TopoJSON](https://github.com/mbostock/topojson) encoded with binary [MessagePack](http://msgpack.org) for parsing speed and intended for use with larger, metatile payloads and potential whole-world renders. This pull is in-progress.

Todo:

* [ ] Add unit tests.
* [ ] Decide whether to stick with geographic coordinates (see commit 6baaa0bc7612)
* [ ] Benchmark encoding speed.