Add a method to prepare the client with basic authentication

[akefirad]

Since basic auth is quite well-known, I'd suggest to have a helper method like before:

    def setup() {
        gru.basicAuth('some-username', 'some-password')
    }

Currently I implement it using an extension method:

    static TestDefinitionBuilder basicAuth(TestDefinitionBuilder self, String username, String password) {
        return self.command(HttpMinion) {
            def cred = "$username:$password".bytes.encodeBase64().toString()
            requestHeaders.put('Authorization', ['Basic ' + cred])
        }
    }

A small improvement, but I'd say it's worth it. LMK if you need a PR.