Sign back in to last team on relaunch

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.




The method simply generates the iframe to dynamically set the url that is initially loaded. If we have a saved a subdomain ( a slack team ) cached, on launch we need to load that team's subdomain into the view.