Add NotifyClosed for signaling when hijacked connections are closed.

Hijacked http connection no longer notify the server of changes to the
connection via the ConnState callback. NotifyClosed will place the
connection in the remove channel so that it is recorded as closed.
Merge pull request #28 from benburkert/notify-closed

Add NotifyClosed for signaling when hijacked connections are closed.
Remove unnecessary variable
Fixes for the ShutdownInitiated callback.

Call the ShutdownInitiated callback after the server has closed the
listener socket. Also removes the unecessary hook variable and switches
the callback's docs to a websocket example.
Merge remote-tracking branch 'origin/master' into shutdown-callback

Conflicts:
	graceful_test.go