Re-organizes ajax.js: prefilters and transports are no longer stored …

…in ajaxSettings (their structure is not handled correctly by extend() and was causing some overhead when constructing the final options map in ajax()); base function for ajaxPrefilter and ajaxTransport has been renamed and split in two (one for inspection, one for addition); response/dataType determination and data conversion logics have been externalized from the ajax() internal callback; data conversion no longer sets responseXXX fields; some minor re-formatting and simplifications.