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
move credit display to frame state
remove credit name
cleanup
refactor
throw if credit is undefined
helper functions for credit
add destroy to credit display
don't add credit if it has already been added as a default
Merge branch 'master' into credits
updated CHANGES.md