Define stormpath.showLogin() behavior to be a better developer experience when the end-user is logged in

[omgitstom]

Adding an issue for this, but we need to have a discussion about this. It isn't a normal paradigm to show

Right now, we show a message that you are already logged in.

Explicitly invoking login should either do:

• Allow the user to login with different credentials (show the login page as normal)
• Fire an event that you can listen to prevent or allow the login screen to show