Merge pull request #536 from gbeatty/video

manually remove `seeked` event listener from old video
Merge branch 'master' into video
video material uses hybrid seek/play for animation
Merge branch 'video' of github.com:gbeatty/cesium into videoWithOverlay

Conflicts:
	Source/DynamicScene/DynamicVideoMaterial.js
fixed animation rate bug
Merge branch 'video' of github.com:gbeatty/cesium into videoWithOverlay
add new test videos
updated test files