Fix bango logout (bug 1105076,1105076,1049004)

* Use logged_in_user as the key for provider logouts. 
* Set the localStorage to a logout+timestamp when we logout (This causes the provider logout later at the point we know the provider).

r? @kumar303 @washort 
if ``this.storage.getItem()`` returns an empty string (or whatever it returns when there's no value), will this throw a parse error?