Event handler fix, new callback and public methods, updates to documentation

This patch includes a fix for callbacks that did not get fired for FB login/logout buttons, as those prevent events that only custom buttons previously got.

We've added a callback handler for situations in which the FB authChange results in a not_authorized status, providing means for developers to show custom message to users in such case.

Public login() and logout() methods have been introduced for the class to both avoid code repetition and to allow developers to call the necessary functionality in custom situations.

Some new docs have been added for the defaultInitOptions property, which is used to extend the options passed to the constructor method.