Remove test for iframe.contentWindow

Remove the check for that.iframeObj.iframe.contentWindow before call postMessage; the utils.createIframe does not return a reference to the iframe (reference removed in change #55 to dom.js) so this check always fails and no close message is ever sent.

The post method set up in utils.createIframe already tests for iframe.contentWindow so this approach seems safe.
Merge pull request #162 from DustyHarry/master

Remove test for iframe.contentWindow