[WFLY-266] Notification Support

* add notifications to AS7 resources description
  notifications are described in :read-resource-description operation if
  the notifications boolean is true
* NotificationDefinition let any resources describe the notifications
  they can emit by registering them in the ManagementResourceRegistration
* add NotificationRegistry (exposed by the ModelController) to
  register/unregister notification handlers
* add NotificationSupport to emit notifications during a managemenet
  operations (through OperationContext.emit(Notification))

Notifications emitted by OperationContext.emit(Notification) are
effectively sent at the end of the operation execution if it is
successful. This ensure that the order of notifications emitted by a
single OperationContext will be received in the same order (unless they
are emitted during the ResultHandler execuion).

If the NotificationSupport is created with an ExecutorService, the
delivery of notifications is non-blocking and will not block the
remaining code path of the operation execution.

JIRA: https://issues.jboss.org/browse/WFLY-266