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fix for issue https://github.com/wlaurance/slack-for-linux/issues/4. 

Whenever we get a 'document-start' (https://github.com/rogerwang/node-webkit/wiki/Window#document-start) event we cache the sub-domain in localStorage. If the domain is a plain slack domain we assume a logout. Upon start-up we append the iframe with the cached sub-domain as the source url.