'idle' event, removeAllListeners(), nbRunning()

• Added the idle event. It is emitted when both nbQueued() and nbRunning() drop to 0, which means there is nothing running and nothing queued up.
• Added the removeAllListeners() method which follows the same pattern as the standard Node.js method of the same name. It supports an optional event name as first argument.
• Added the nbRunning() method which returns the number of requests currently running in the limiter.