First half of graceful stop: network accounting.

This is pretty standard and could have just been cribbed from Keith's or anyone else's examples.
Second half of graceful stop: killing keepalives.

This adds a channel and a ResponseWriter implementation to preemptively close connections (by injecting a Connection: close header) when the listener's closed.
Documenting graceful stop slightly.
At Brad's suggestion, combine Close and Wait.

Now Close blocks until all connections have closed.
Merge branch 'graceful-stop'

Conflicts:
	server.go