Bug 923054 part 1 - Convert DataTransfer to WebIDL bindings, move files and rename class.  r=smaug

--HG--
rename : dom/events/nsDOMDataTransfer.cpp => dom/events/DataTransfer.cpp
rename : dom/events/nsDOMDataTransfer.h => dom/events/DataTransfer.h
Bug 923054 part 2 - Convert DataTransfer to WebIDL bindings, add WebIDL API and switch to the WebIDL binding.  r=smaug