Issue#18 : Create a 'server_client' to allow 2 server talk each other

It's a simple implementation of a "ForceClient" for the server side.

This allow this kind of code:

int main() {

ForceClient fc = new ForceClient(host: "127.0.0.1", port: "9223");
ForceServer fs = new ForceServer( port: 9223);

    fs.start().then((e) {
      fc.connect();
      fc.onConnected.listen((e) {
        fs.on("test", (e, sendable) {
          print("Receive from server: ${e.json}");
          sendable.send('test', e.json);
          fs.close("Stop");
        });
        fc.on("test", (e, sender) {
          print("Receive from client: ${e.json}");
        });
        fc.send('test', "Trololo");
      });
      fc.onDisconnected.listen((e) {
        print("Deco: ${e}");
      });
    });
  }