removed async, style formatting, error fixes

I removed the need for the async library (I wasn't using it for anything else for my project - this should not be a dependency for a small login task). 

The window.activeSession.clear() is problematic because it completely deletes the attributes (doesn't just clear their values). This causes problems when I log out and log back in during the same session.  Just set values to null instead. 

use has() method. It returns a boolean if the attribute value is not null. No need for Boolean() call. 

Also added a missing result param in the method onError().

+ style changes according to Airbnb style guide. Much cleaner. 

Thanks for the awesome module man!