strconv: add example for Unquote.
strconv: add example for Unquote.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/38920043

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang