Add graceful shutdown to TcpServer

This commit prototypes support for graceful shutdown in `TcpServer`
through some new methods. A `TcpServer::bind` method was acced which is
used to construct a server and unlike `serve` returns the value
immediately.

The returned value can then be used to inspect the state of the server,
finally calling `run` to execute indefinitely or `run_until` to run
until a shutdown signal is received.

src/lib.rs

@@ -199,7 +199,7 @@ mod tcp_client;
 pub use tcp_client::{TcpClient, Connect};
 
 mod tcp_server;
-pub use tcp_server::TcpServer;
+pub use tcp_server::{TcpServer, TcpServerInstance};
 
 use tokio_core::reactor::Handle;
 use tokio_service::Service;