Extracted some constants ("connectionId" and "subscriptionId")
Made the whole addSubscription() method synchronized (probably wasn't
thread-safe before)
Added unsubscribe
SubscriptionManager now stores logins instead of (Channel) IDs, since
Channel ID becomes invalid when the client disconnects
Merge branch 'development-unsubscribe' into protocol-tests
Fixed ClassCastException
Added some unsubscribe tests

Added a SimpleMessageWrapper class
Merge branch 'development' into protocol-tests
Merge branch 'protocol-tests' into protocol-tests-unsubscribe
Fixed NPE when there was no next state (transition)
Merge branch 'development' into protocol-tests
Merge branch 'protocol-tests' into protocol-tests-unsubscribe