Better error reporting when attempting to publish a non-serializable event

[jhalterman]

Copycat allows non-serializable events to (attempt to) be published, which appears to result in the event never being received by the client and the server attempting to resend it over and over.

Possible solution, throw an exception (IllegalArgumentException?) when attempting to publish a non-serializable event. Also update docs to more clearly state that all event types must be serializable (by Java or Catalyst).