Missing Event::DISCONNECT

[zeleznypa]

At first, I want to thank you for such a great database layer.

In my case it will be useful to have a possibility to have the Event::DISCONNECT or the onDisconnect event.

Would you mind @dg if I create a pull request with this change, or you think that it is something wrong?

dibi/src/Dibi/Connection.php

Lines 180 to 181 in 04f006a

	$this->driver->disconnect();
	$this->driver = $this->translator = null;