Show view-specific Bing attribution.
Better mechanism for obtaining credits.
Handle the possibility that no attribution is provided.
Add getTileCredits to all imagery providers.
Update tests with updated URL.
Remove useless comment.
Move terrain/imagery credit update to surface.
Merge remote-tracking branch 'origin/master' into bingAttribution

Conflicts:
	CHANGES.md
	Source/Scene/CentralBody.js
	Source/Scene/CentralBodySurface.js
Make credits smaller and don't wrap them in the middle.
Re-add provider-level credits.
Update CHANGES.md.
Merge pull request #1338 from AnalyticalGraphicsInc/bingAttribution

Display attribution for the Bing Maps tiles displayed in the current view.