Improve CORS handling

Found this issue when troubleshooting TinCan_Prototypes. In IE < 8 we didn't fail very gracefully, this improves that by indicating through 'alert' (our standard way) to the user that they are trying to do a CORS transaction in a browser that doesn't support it. This should also be leveraging IE 10's new handling of the native method by feature detection on XmlHttpRequest rather than just detection of XDomainRequest.

Tested in IE 7, 9 by me through Golf Example and by @tseabrooks in IE 8.