initial attempt
update credit changes in viewer
separate images and text credits
style cleanup
style tweak
documentation and tests
fix tests
rename
fix documentation and style
Merge remote-tracking branch 'agi/master' into credits

Conflicts:
	Source/Scene/CentralBody.js
	Source/Scene/TileMapServiceImageryProvider.js
	Source/Widgets/CesiumWidget/CesiumWidget.js
developer error if no container is provided for credit display
refactor ellipse computation into functions
move credit display to frame state
remove credit name
cleanup
finished extrusion
tests
circle geometry extrudes correctly
refactor
throw if credit is undefined