Remove support for geojson.io

There isn't much need for this anymore. The idea origianlly was to
use geojson.io to generate different output formats - issue #6, this
is now all done in archmap.py though.

It seems like a lot of extra baggage to keep around for cutting out
just 2 small steps:
* Opening up http://geojson.io/
* Clicking `Open` and importing the GeoJSON
Fix license badge capitalisation