Make event handlers optional to define

Ohai there!

Thanks for this little Gem. I used it for my [@Horse_diceware](https://twitter.com/Horse_diceware) Twitter bot. :)

I ran into some problems though because I assumed that the event handlers were optional to define, so my bot crashed because it attempted to call the  `#call` method on Nil objects.

I added some simple checks to ensure that the event handlers are defined before attempting to invoke them.

Cheers.