Add meck:capture(..., result)

[rlipscombe]

See #249

I'm not sure whether we want to deprecate the use of meck_history:result(), so I only changed one of the unit tests to use the changed meck:capture().

Let me know what direction you prefer, and I'll change the PR.

[eproxus]

@rlipscombe Thanks! Since result already exists and has the signature it has, I don't think it is too much of a stretch to allow returning the return value of a call too. What do you think about changing it to return_value instead? That matches the language in the Erlang documentation more.