mass refactoring of $.attr (#2548)

* Changes
- undefined was returned for falsy values.
- expando attributes are used when possible.
- one $.isXmlDoc instead of 2.
- $.attr( style, ... ) goes thru less useless checks.
- reduced code size of recurrent accessed variables/checks.
- $.props doesn't need redundant sets of key value anymore.
- added cellSpacing to $.props (#2521)

jquery core: closes #2811, $.fn.add now calls $.unique internally, also it was failing for array-like f.e: window.

$.inArray now makes a === check because of IE.
$.inArray is used in $.fn.index, this is shorter, and breaks the loop when possible.
$.fn.index can receive a jquery object, and the first element is used

test runner: making 2 tests more flexible because they yield a slightly different result on Safari 3, but still acceptable.

jquery ajax: support for dynamic data arguments, see #2806; added test/data/echoQuery.php for easier testing of sent query strings

test runner: translated all the ok() with '==' to equals(), as it gives more information on failures.

jquery testrunner: reset global events after each run, speeds up a test run for ajax from 16s to 10s (test/?ajax), kudos to Ariel