add support for returning all data in runClientMockup

This feature is usable to test a whole transaction session. If you just get the last response there might be a problem in the middle that you do not find. E.g.;
> S: 220 HI
> C: HELO
> S: 250 OK
> C: MAIL FROM:\<a@a.a\>
> S: 250 OK
> C: RCPT TO:\<b@b.b\>
> S: 550 <-- you will miss this one for example
> C: RCPT TO:\<b@b.a\>
> S: 250 OK

The API change is trivial; adding a second parameter to the callback. By doing so, we have full backward compatibility.

My tests passes in all versions specified in `.travis.yml` except in v0.11.6 where nothing seams to work due to `createSecurePair() is deprecated, use TLSSocket instead`.

I have tried to follow your coding style, but please fix/tell me what is wrong if you are not satisfied.

**Commit message;**
* return a second argument in the callback with all the responses
* tests
* documentation