Adding support for XMLHttpRequest.upload to FakeXMLHttpRequest

I've added support for HTML 5 upload events to the FakeXMLHttpRequest. Here's an issue requesting discussing functionality:

https://github.com/cjohansen/Sinon.JS/issues/185

Please let me know if there are any changes I can make, to get you pumped about pulling this in :thumbsup: 

__Reading__

* MDN article discussing upload functionality: https://github.com/cjohansen/Sinon.JS/issues/185
* RFC outlining progress events: http://www.w3.org/TR/progress-events/#progresseventinit