Return a requestID from requestAnimationFrame, add cancelAnimationFrame.

This makes our shim match the standard implementation in features.
Merge pull request #880 from cmorse/master

Some documentation fixes
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
Merge pull request #878 from AnalyticalGraphicsInc/requestAnimationFrame

Return a requestID from requestAnimationFrame, add cancelAnimationFrame.