Implement Socket.io interface

Enable Socket.io on PgREST

    pgrest --db test --websocket

All REST api is implemented on websocket

    <script src="http://HOST:PORT/socket.io/socket.io.js"></script>

    <script>
    var socket = io.connect('http://HOST:PORT');
    socket.on("complete", function (data) {
      // data = REST API return value
    });
    socket.emit("GET:foo");
    </script>

More importantly, you can subscribe to a collection to be notified when it's being inserted

    var socket = io.connect('http://HOST:PORT');
    s.on("CHANNEL:foo", function (data) {
      // called every time something is inserted into foo
    });
    s.emit("SUBSCRIBE:foo");

test [included](test/socket.ls)

Authentication is not implemented yet. I think I should wait [this issue](https://github.com/clkao/pgrest/issues/35) to complete and do some follow-up work?