Implement complete close handshake

I changed my mind after #103 as browsers include a wasClean event to indicate
whether the connection was closed cleanly. From my tests, if a server using
this library prior to this commit initiates the close handshake, wasClean
will be false for the browser as the connection was closed before it could
respond with a close frame. Thus, I believe it's necessary to fully implement
the close handshake.

@stephenyama You'll enjoy this.