better support for handling responses from ajax upload

I'm all for a better route to do this if you have any ideas, but we needed a fix and I found that `jquery.iframe-transport.js` was mishandling our responses and thus ignoring them completely.  Since we're already using jQuery, we might as well make use of some of the features that help to more dummy-proof the data handling. 

I also changed the response-code attribute to data-response-code for obvious reasons.