Fix for XBlock/Studio/time bug

@dmitchell @cpennington I was using XBlocks in Studio, and getting funny import/export errors with XML content. This fixes it. I do not understand the code path around this basically at all. The fix was purely mechanical -- I saw strings going in and matching exceptions, so I made it handle strings. I'd appreciate a more detailed look-over to see if this is sane. 
I don't see any code in the AnimationXBlock that is using the `DateTuple` field type. In fact, I also don't see any code in edx-platform that is using that field type, either. Perhaps we can just delete this class entirely?