First pass at exporting the map to png

This uses a few JS libs to convert the SVG layer (i.e. census tracts, bubbles, etc.) into a canvas, which can then be exported to a png.

Right now it opens a popup (may be annoying) and does not include the underlying tile set, nor the key:

![screen shot 2014-08-08 at 6 27 31 pm](https://cloud.githubusercontent.com/assets/326918/3863607/77f64c86-1f4b-11e4-98bb-23d86378d09b.png)
![exported](https://cloud.githubusercontent.com/assets/326918/3863608/7d6ddc2e-1f4b-11e4-8ccc-7a4b3ed6cf9e.png)