go-tour: set PlayEnabled flag before parsing
go-tour: set PlayEnabled flag before parsing

initTour is called from init funcs in local.go and appengine.go,
while PlayEnabled was being set by an init func in tour.go.
It was possible that initTour would be called, rendering the tour,
before PlayEnabled is set to true.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7409043

BenjaminTsai/go-tour-zh-tw,BenjaminTsai/go-tour-zh-tw