Promise shouldn't be reused

With current code, deferred object is created only once and is then reused. This can have multiple issues, especially if you are building single page apps.

For example, I could login with one account, use app for some time, log out from web app and g+, try to login with different user, but defered object would still resolve first login.

I think this issue (#7) will be resolved with this fix as well.