math/big: add examples for Rat and Int's SetString and Scan methods
math/big: add examples for Rat and Int's SetString and Scan methods

R=golang-dev, bradfitz, rsc, r, gri, r
CC=golang-dev
http://codereview.appspot.com/5543047

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go