Golang client test with s3gof3r

Add Go lang to client test lineup.

Add only one test case, which used s3gof3r and confirm
Content-MD5 of Complete Multipart Upload response and

Not like other client tests, test code is not implemented in Go but
just some lines in `Makefile`. This is because s3gof3r provides a
command line tool gof3r and go has standard way of placing libraries.

Of cource, the Go test target is added to top level `Makefile` and can
be executed both by `make test-client` and by riak_test
`external_client_tests`.

This PR addresses #889.