Creates the appropriate XDR transport for this browser.
- IE 7 and below don't support CORS
- IE 8 and 9 support CORS with native XDomainRequest object
- IE 10 (and above?) supports CORS with native XMLHttpRequest object
@private
function() {
        var xdr;

        if (Ext.ieVersion >= 8) {
            xdr = new XDomainRequest();
        } else {
            Ext.Error.raise({
                msg: 'Your browser does not support CORS'
            });
        }

        return xdr;
    }