fix #970 aria.utils.Date.format can now handle falsy values and empty objects

It now returns `null` in those cases instead of failing with a `TypeError`.

Close #971.
fix #969 SynEvent used pageYOffset and pageXOffset on all version of IE

These properties are not supported on IE <= 8
(http://www.quirksmode.org/dom/w3c_cssom.html#windowview),
and returned undefined.

Now a fallback on scrollTop and scrollLeft has been done if
these properties doesn't exist.

Close #898, close #664, close #635, close #663