go-tour: trim spaces around source code before displaying
go-tour: trim spaces around source code before displaying

Changes to the way code is generated add some space around the
code snippets, which can be quite ugly. This fixes that issue.

R=golang-dev
CC=golang-dev
https://codereview.appspot.com/10729043

Go-zh/tour-old,minux/go-tour-zh,minux/go-tour-zh,Go-zh/tour-old