Support for events

For several use cases it would be quite convenient if JASPIC would throw events at several important moments of the authentication message exchange.

Such events could be:

* PreAuthenticate
* PostAuthenticate
* PreLogout
* PostLogout

User code could possibly register for such events in [the same way](https://javaee-spec.java.net/nonav/javadocs/javax/servlet/http/HttpSessionIdListener.html) such code can register for events from the Servlet container; annotating the listener class and implementing an interface.

E.g.

```
@SecurityListener
public class MyListener implements AuthenticationListener {

public void preAuthenticate(AuthEvent authEvent) {
// ...
}

public void postAuthenticate(AuthEvent authEvent) {
// ...
}
}
```

Additionally CDI style events can be supported as well.

Use cases for such event listeners are among others:

* Keeping track of the number of logged-in users
* Protecting against brute-force attacks by keeping count of failed login attempts for a certain account
* Creating a new local user after the first successful authentication via a remote authentication provider
* Loading application specific preferences into the HTTP session after a user logs-in

Specifically for the second use case a PreAutenticate listener should be able to veto the authentication attempt (at which JASPIC could respond by e.g. sending a 403 to the client).