Fix Billboard.computeScreenSpacePosition.
Update CHANGES.md.
Merge branch 'master' into computeScreenSpacePosition

Conflicts:
	CHANGES.md
Update CHANGES.md.
Merge branch 'master' into computeScreenSpacePosition

Conflicts:
	CHANGES.md
	Source/Scene/Billboard.js
	Source/Scene/Label.js
Fix after merge.
Update based on review.
Revert BillboardCollection and update CHANGES.md.
Add comment to computeScreenSpacePosition
Merge pull request #557 from AnalyticalGraphicsInc/computeScreenSpacePosition

Fix Billboard.computeScreenSpacePosition.