Further refinement of my [d3-slippy module](https://github.com/Andrew-Reid/d3-slippy/blob/master/README.md) to display geographic tiles/web map tiles. 

This block runs through all the built in tilesets that can be accessed by string. I'll be changing how the tilesets are represented, but for the moment this serves as a test and demonstration of the various tilesets out there. There appear to be a few issues with some of the tilesets at certain zoom levels in certain areas, still a few kinks to sort out.

I'm not quite happy with the module's structure yet, but getting closer. Key challenges are implmenting clean non-cylindrical tile sets, this is causing some grief in terms of positioning.

