Provide emit method for Pool module

[martynsmith]

This provides the same emit method as implemented in the Socket class to the Pool class.

[martynsmith]

Yes, except you can't do it easily from the Pool.

My problem is that I have something generating events that isn't in the context of any one connection. An external event (i.e. some other AnyEvent code I've written) is generating the events, so I don't have a Socket handle when I want to emit them. Instead I'm using $pocketio->pool