Mockbroker

Semantics of the tests are identical (AFAIK), but I added an abstraction to generate trivial responses.

It's a bit duplicated from the main package, but there were only a few bad options here:

1) Expose a bunch of internals;

2) Link a bunch of testing API bits into the library;

3) Duplicate some code to generate requests.

Anyway, the major win here is that other apps can import `mockbroker` to test against mock brokers the same way sarama now does. This is something a bunch of different projects have been struggling with here.

@eapache @fw42 @wvanbergen