Support topoJSON

[wilson428]

Thrilled for this repo. Mike Bostock offered up some wonderful instructions on how to get topoJSON directly from SHP files:

https://groups.google.com/forum/#!topic/d3-js/a6VdcMv0VmU

[konklone]

That's terrific. I've added topoJSON to the list of formats I want here. Is there a us.topojson you have from that project you'd like to contribute?

[wilson428]

Sure, will do. The baked-in population data is super useful.

I actually use QGIS to clip the political boundaries to the land boundaries
so that Michigan doesn't look so weird. This violates Bostock's "Make all
the way down" (http://bost.ocks.org/mike/make/) tenet, but I find ogr2ogr
to be maddeningly slow and poorly documented for this task.

[konklone]

I've just added GeoJSON for all federal CDs, and states, and reshuffled the URLs around a bunch and updated the docs.

[konklone]

Just found Simplify.js for polygon simplification, that may be helpful.

[konklone]

Actually, us-atlas looks like the winner here.

[atticoos]

+1 for topoJSON

[bradoyler]

FYI. I've added Topojson via #6
Looking for feedback