Add vector tile TopoJSON output

This pull is in-progress, and implements a simple TopoJSON encoder for output from VecTiles. I’m building from the [specification and linked examples](https://github.com/mbostock/topojson/wiki/Specification).

To do:

* [ ] Confirm correct output for point, line and polygon geometry types with Mike B.
* [ ] Use scale and translation features to switch from floats to integers.
* [ ] Add support for multi-geometries.
* [ ] Test with all [vector-datasource](http://openstreetmap.us/~migurski/vector-datasource/) layers for completeness.
* [ ] Move TopoJSON-related code to its own module under VecTiles.