Map should read/write geojson to and from github gists

[jaycrossler]

• [ ] Pages should be able to export to geo-json gist formats: https://github.com/blog/1576-gist-meets-geojson
• [ ] Layers should be able to point to a geojson gist to read and show it
• [ ] Possibly there should be a place to copy and paste in new data in kml/geojson/gist/georss format to quickly add onto the page.