This reverts commit c22330fd.

This enables access to all UserSessionService objects from within the
application. This could be used to send events to all session users.

This patch uses an in memory storage of logged in users. Every time
users send a (online) heartbeat the last known active timestamp of all
users will be checked and inactive users will be removed from active
user list.

This could be used to send data via websockets without searching
sessionIDs or user tables within ARSnova.

Simply publish an event with ARSnova-sessionKey from within the
application.

This sets current user and ARSnova session information in a session
scoped service bean. If the user connects with a websocket, the
websocket session ID will be set in addition to the user and session
object.

The aspects methods pointcut after returning from joinSession() method
in SessionService. So the UserSessionService will be set with all needed
data after a user successfully joins a session.

With this service available, it can be used to replace some usage of
Array/Map based application wide data structures that cause some trouble
when stopping or redeploying the application.

This bean holds user and session object of the current user after he
joins a session and could be used to access those objects within the
application.

Added two simple URIs to get a JSON representation of the referenced
objects.