Add the ShutdownInitiated callback to Server.

The ShutdownInitiated callback is called at the beginning of the
shutdown process and is useful for logging the shutdown action.
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
Merge pull request #27 from benburkert/shutdown-callback

Add the ShutdownInitiated callback to Server