Implement service discovery

[robberphex]

1. Connect to a host:

   let builder = ClientBuilder::from_static(&"http://127.0.0.1:8888").with_timeout(1000000);
   let mut cli = EchoClient::new(builder);

2. Or use customized directory:

   let builder = ClientBuilder::new();
   builder=builder.with_directory(customized_directory);
   let mut cli = GreeterClient::new(builder);

[yang20150702]

implementation of RpcInvocation is too complicated. Now we can use a struct as metadata. In future, we can rebuild this struct by new requirements

[yang20150702]

Dubbo的一个特性是配置优先。我建议可以将服务发现相关的配置放到clientBuilder中，通过配置来启动服务发现；这样可以简化dubbo-build侧client生成的代码
另外，自定义配置directory应该是高级能力。

[AlbumenJ]

remove