lua: add missing notification callback

lua: add missing notification callback

* register_notification

In this version, it support callback function something like:

  function notification(notif)
     print(inspect(notif))
     return 0
  end

  collectd.register_notification(notification)

It accepts PUTNOTIF event via plugin_dispatch_notification then you can
handle it.

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>