Add an animation to provide feedback for asynchronous CZML loading.

I got tired of the lack of feedback indicating whether the viewer was in the process of loading CZML or not, so I created an animation with http://www.ajaxload.info/ and overlay it whenever we're waiting on an asynchronous load.  I also noticed potential race conditions if you drag and drop 2 files in quick succession, but I'll fix that in another pull.