jquery core: #2548, #2170, #2188, #2099, #1170, #2558, #2521, #2119, #1271, #2453...

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)