bug fix for IE8 without z-index and overlayLayer's onExit when onClick i...

Hello. Thanks for such a great javascripts!
I found some bugs so send a fix request.

1. when 'exitOnOverlayClick:false' and set 'onexit' callback, every time clicking overlay layer causes unnecessary onexit callback.


2. when IE8, some conditions returns 'z-index' propValue as undefined, and toLowerCase method results in like null pointer exceptions.


please check it.