Initial CZML overlay quad support
renamed DynamicOverlayQuad to DynamicScreenOverlay
Merge branch 'ViewportQuad' of https://github.com/gbeatty/cesium into czmlOverlay
fixed material uri lookup
Merge branch 'master' of https://github.com/gbeatty/cesium into czmlOverlay

Conflicts:
	Source/Scene/ViewportQuad.js
Fixed bad merge
Added new tests for `DynamicScreenOverlay`
updated `CHANGES.md`
Merge branch 'master' of https://github.com/gbeatty/cesium into czmlOverlay
moved creation of screenOverlay material to prevent unnecessary allocations
Merge branch 'master' of github.com:gbeatty/cesium into czmlOverlay

Conflicts:
	CHANGES.md
Merge branch 'master' of github.com:gbeatty/cesium into czmlOverlay

Conflicts:
	CHANGES.md
	Source/DynamicScene/CzmlDefaults.js