Return a requestID from requestAnimationFrame, add cancelAnimationFrame.

This makes our shim match the standard implementation in features.
Added getTilingScheme to GoogleEarthImageryProvider
Fix crash in Viewer

Clicking on a DynamicObject without a position property causes an exception.
This is a regression introduced in the Viewer refactor.
Merge pull request #881 from AnalyticalGraphicsInc/viewer-crash

Fix crash in Viewer
Fix #879

Allow users to specify `options.imageryProvider` rather than `options.selectedImageryProviderViewModel` when the BaseLayerPicker is not being used.
Merge pull request #878 from AnalyticalGraphicsInc/requestAnimationFrame

Return a requestID from requestAnimationFrame, add cancelAnimationFrame.
Merge branch 'master' of https://github.com/AnalyticalGraphicsInc/cesium into googleEarth
Merge branch 'issue879' of https://github.com/AnalyticalGraphicsInc/cesium into googleEarth