Fix problem of FakeXMLHttpRequest via $.ajax with async:false

add support for $.Deferred of jsXHR(ex: jqXHR.done()) with async:false

In [specification of XHR](http://www.w3.org/TR/XMLHttpRequest/#switch-done), xhr fires readystatechange/load event even when synchronous flag is set.
And jQuery.ajax is depending on whether load event is fired from xhr in its callback handling.
So, it might be better that FakeXMLHttpRequest behave as same as when async:true even when async:false, I think.

(related: #510 )