Merge remote-tracking branch 'upstream/develop' into develop

# Conflicts:
#	README.md
#	src/config/worldwind.layers.xml

Implemented a naming convention for USGS topographic map layers:

- USGS Topo Base Map
- USGS Topo Base Map Large Scale
- USGS Topo Scanned Maps 1:250K
- USGS Topo Scanned Maps 1:100K
- USGS Topo Scanned Maps 1:24K

Initial commit

Initial commit

Initial commit

Initial commit

Initial commit

Initial commit