Check the received value in producers (synch/asych) mocks agains a regexp

[jacksilv]

Fixes #667. Thanks to have a look guys

[eapache]

I would prefer not adding a method to Encoder, not all messages are encodable as strings. Instead I think the more general solution is to support passing a callback func ([]byte) error into the mock. Then if you know your messages are strings you can cast yourself and do whatever regexp checks you want; if not you can do whatever other sanity checks you want. Does this make sense?

[jacksilv]

Thanks @eapache for your comments, I updated my code

[eapache]

could use a (brief) godoc comment

[eapache]

Thanks, this is pretty good now, just a few little things.