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 
This needs to unset the value. I'm not sure if it's a promise or not, but maybe:

    return this.logout().then(function() {
        this.storage.getItem('needs-provider-logout', false)
    });