Optional graceful close specified by WebSocket constructor

[pgriess]

It seems that nobody implements graceful close on the server.

We should default to non-graceful close (i.e. sever the TCP connection), but allow clients to specify that they wish for graceful close using an option in the WebSocket constructor.

[rauchg]

+1

Let's sever the TCP connection with .close, and let's send the close packet with .close(true).