Netutil base protocol implementation

If we will use ``netutil.TCPServer`` for implementing some functionality, we will have to use inheritance and write duplicated code more and more times... (example of such application you can find here https://gist.github.com/1387470). 

The keynote here that there are many situation when we don't want to create server class (inherited from ``netutil.TCPServer``), we just want to use custom handler as wrapper for  incoming ``IOStream``. We can easily avoid this problem, code example in docstring to ``BaseProtocol``. Duplicate this example here:

    class EchoProtocol(BaseProtocol):
        def _on_connect(self):
            self._wait()
              
        def _wait(self):
            self.stream.read_unit(b('\n'), self._on_read)
              
        def _on_read(self, line):
            self.stream.write(line, callback=self._wait)

    server = TCPServer(protocol=EchoProtocol)
    server.listen(8888)
    IOLoop.instance().start() 

It also possible to implement this approach for ``httpserver.HTTPConnection`` (request_callback, no_keep_alive and xheaders can be read from server param). But I don't know if it will bring many benefits (only one - standard approach).