move central body
add imageryLayer, terrainProvider to scene
CentralBody -> Globe
update sandcastle
update CHANGES.md
Merge branch 'master' into centralBodyToScene
Tweak CHANGES.md
change imageryLayers and terrainProvider in demos
replace 'central body' in comments
Merge pull request #1637 from AnalyticalGraphicsInc/centralBodyToScene

Single Scene entry point: CentralBody