Ajax: Integrated native datatype, for XHR2 rich responseType support

XHR2 provides a response attribute, that contains a converted response to a native object, depending on the responseType ("arraybuffer", "blob", "document", "json"). See:
https://dvcs.w3.org/hg/xhr/raw-file/tip/Overview.html#the-response-attribute
http://www.html5rocks.com/en/tutorials/file/xhr2/?redirect_from_locale=fr

I propose in this pull request, to create a native datatype and response, to manage this kind of response. Some of previous closed tickets proposed to include a binary dataType, with adhoc changes on the transport function :
http://bugs.jquery.com/ticket/11461
http://bugs.jquery.com/ticket/9992
http://bugs.jquery.com/ticket/14068

I think it is not appropriate as the response attribute deals with more than binary data, like arraybuffer, but extends to blob, document and json datatype. In my pull request, I create a native datatype to support all the four response types that are abstracted.

I think also that such improvements should be included as this is a new improvement in XHR, that is not already included in jQuery. Also, trying to use a converter in the case of ArrayBuffer for example, leads to a heavy loop for the conversion (See: http://updates.html5rocks.com/2012/06/How-to-convert-ArrayBuffer-to-and-from-String). Simple casting doesn't function.

In my case, I have used Brain dMRI NifTI data formats that are 20MB in size, and all of them need parsing after downloading, which is also a heavy computation (a hundred of milliseconds).

Also, treating a binary data as text, it is just not natural at all, as this is already the case.

In any way, I think we should find a solution, and this is my proposition. Please, take a look at the tests, for the usage. And let's start a discussion around this.