Connection direction

If userA follows userB, trying to add userB as a follower of userA throws an exception as the users are already connected. I can see the value of keeping bidirectional connections (where whether a node is the source or the destination doesn't matter), for friend connections, for example, but one way connections should also be possible. I've kept the default behaviour to two-ways connections in order not to introduce backward incompatibilities. Also, there might be more changes needed, for now only the Manager::connect and Repository::areConnected are aware of the direction of the connection.