go-tour: Stringer should be an interface, not a struct
go-tour: Stringer should be an interface, not a struct

Fixes #184.

LGTM=minux
R=campoy, minux
CC=golang-codereviews
https://golang.org/cl/183100043

Jasperste/tour,phoorichet/tour,Go-zh/tour,zorion/go-tour-ca,Jasperste/tour,Jasperste/tour,raindev/tour,Go-zh/tour,ybin/tour,shuLhan/tour,scryner/tour,shuLhan/go-tour-id,tamnd/tour,scryner/tour,Go-zh/tour,raindev/tour,scryner/tour,rofrol/go-tour-pl,LaPingvino/go-tour-nl,campoy/go-tour-ua,shuLhan/go-tour-id,zorion/go-tour-ca,LaPingvino/go-tour-nl,LaPingvino/go-tour-eo,rofrol/go-tour-pl,tamnd/tour,LaPingvino/go-tour-eo,bbandaru/tour,dupoxy/go-tour-fr,campoy/go-tour-ua,phoorichet/tour,rofrol/go-tour-pl,LaPingvino/go-tour-eo,scryner/tour,bbandaru/tour,raindev/tour,LaPingvino/go-tour-nl,golang/tour,bbandaru/tour,tamnd/tour,golians/tour,dupoxy/go-tour-fr,golians/tour,ybin/tour,phoorichet/tour,dupoxy/go-tour-fr,Go-zh/tour,shuLhan/tour,shuLhan/go-tour-id,ybin/tour,golians/tour,shuLhan/tour,campoy/go-tour-ua