Added US States GeoJSON to Viewer Gallery.

The new data is already covered by this section of LICENSE.md:

### Public domain data from Natural Earth

Free vector and raster map data @ naturalearthdata.com

Terms of use: http://www.naturalearthdata.com/about/terms-of-use/
Upgrade simple.czml to include access availability data.
Merge branch 'master' into gallery
Update simple.czml

Removed Landsat and added some better descriptions.  File is much smaller now.
Tweak description.
Merge pull request #1415 from AnalyticalGraphicsInc/gallery

Gallery file upgrades