time: add After

time: add After

Permits one to easily put a timeout in a select:

select {
case <-ch:
	// foo
case <-time.After(1e6):
	// bar
}

R=r, rog, rsc, sameer1, PeterGo, iant, nigeltao_gnome
CC=golang-dev
https://golang.org/cl/2321043