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.




None of the other vars are equals based aligns. Please make this consistent with:

```js
var slackLoginUrl = 'https://slack.com/signin';
```