Fix #15173: Support `responseURL` attribute for jqXHR

As I reported in [#15173](http://bugs.jquery.com/ticket/15173), `responseURL` attribute is defined in WHATWG XMLHttpRequest spec and is already implemented in Firefox Aurora and Chrome dev channel.  However we can not get that attribute information from `jqXHR` currently.
This pull request adds `responseURL` attribute for jqXHR.

Tested browsers:
* Chrome:
  * Stable channel (Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/35.0.1916.153 Safari/537.36)
  * Dev channel (Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/37.0.2062.3 Safari/537.36)
* Firefox
  * Stable (Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:30.0) Gecko/20100101 Firefox/30.0)
  * Aurora (Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:32.0) Gecko/20100101 Firefox/32.0)
* Safari
  * Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_3) AppleWebKit/537.76.4 (KHTML, like Gecko) Version/7.0.4 Safari/537.76.4