Adds full response object as third argument to rpc callback

[jonbretman]

First of all great work on including a generated client - makes life much easier.

This adds the full response as a third argument to the callback which is useful if you need access to response headers/trailers.

[jonnyreeves]

Thanks for raising this patch.

In a similar vein to the discussion in #59, I'm not currently comfortable with us breaking away from the pattern/API present in the official node-grpc library.

Note that the official node-grpc API returns a client object from unary invocations which will dispatch an event when the metadata becomes available, perhaps you would consider implementing this in which was I would gladly land it.

Thanks

[jonbretman]

@jonnyreeves makes sense. I'll take a look at doing that but #61 is more urgent 😄