Add SyntheticDragEvent with "dataTransfer" property

Very straight-forward. It doesn't do anything to normalize differences in the `DataTransfer`-interface or the event flow. I *think* there are a bunch of differences that possible could be worth normalize (do we want to normalize `DataTransfer` at all?), but I think that's better saved for another PR if so.

Tested and verified IE9, IE10, IE11, and edge FF + Chrome. (IE9 doesn't support files)

https://github.com/facebook/react/issues/727