removeEventListener(type:string, cb:() => void) {
    log.info('remove event listener', type, cb)
    if(!(type in this.listeners))
      return
    let handlers = this.listeners[type]
    for(let i in handlers) {
      if(cb === handlers[i]) {
        handlers.splice(i,1)
        return this.removeEventListener(type, cb)
      }
    }
  }
Remove an event listener
@param  {string} type Type of the event listener
@param  {()=>void} cb Event listener function.
@return {[type]}             [description]