set binary load to async, and removed the usage of dataType

Discussed previously in the mailing list the issue here. Jason noted the request was oddly set to synchronous, so I changed that, allowing the response type to be changed.  I also removed the usage of datatype since that could not be resolved. Works with the current test implementation of melonjs-spine repo. 

Please review, and let's discuss if this works. My main concern is the change to dataType, as I wonder what that resolved to before.