Add async message producer interface

[codeliner]

See prooph/psb-bernard-producer#1

[codeliner]

ping @prolic

[sandrokeil]

What naming conventions are you using? ZF2 uses the suffix Interface for Interfaces. So you know that it is an interface and you don't have to use a new name for a concrete implementation. What do you think about MessageProducerInterface?

[codeliner]

hehe, prolic had the same question a few days/weeks ago. Please see: prooph/event-store#59 (comment)

So you know that it is an interface and you don't have to use a new name for a concrete implementation.

IMO it is an advantage that I have to think about a good name for a concrete implementation.
See: http://verraes.net/2013/09/sensible-interfaces/ .
I absolutely agree with Mathias and it is a great article about the topic.