Add reflection support

[bojand]

Addresses #35.

Now if -proto or -protoset are not specified, we default to server reflection and try to get the method details from server using a reflection request. Optionally -rmd can be used to specify metadata as stringified JSON used only for reflection request.

Example:

ghz -insecure \
    -rmd '{"token": "foobar"}' \
    -call "helloworld.Greeter.SayHello" \
    -d '{"name": "Bob Smith"}' \
    0.0.0.0:50051