jquery core: closes #2930. Normalizing attr() to return undefined, even when getting DOM attributes.

jquery ajax: Closes #2567, additional setting for $.ajax called 'dataFilter'. It's an optional function that receives the ajax response, and returns the sanitized version.

jquery core: SHOULD fix #2802. Caching document.defaultView, instead of defaultView.getComputedStyle as it fails on Safari 2.

jquery ajax: fixed #2865 and #2570. Not passing username to xml.open if it's null, or it open a login popup on some Opera's.

jquery dimensions & offset: moving the local function 'num' to core, so it can be safely used by both modules.

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)