google-api-go-client: add campoy@golang.org to CONTRIBUTORS
google-api-go-client: add campoy@golang.org to CONTRIBUTORS

R=bradfitz
CC=golang-dev
https://codereview.appspot.com/9602043

webhost/google-api-go-client,shibukawa/google-api-go-client,googleapis/google-api-go-client,t9nf/google-api-go-client,shibukawa/google-api-go-client,ropik/google-api-go-client,evandbrown/google-api-go-client,googleapis/google-api-go-client,google/google-api-go-client,stems/google-api-go-client,jfcote87/api2,peterkshultz/google-api-go-client,digideskio/google-api-go-client,odeke-em/google-api-go-client,likaiwalkman/google-api-go-client,krsjoseph/google-api-go-client,ropik/google-api-go-client,google/google-api-go-client,80nashi/google-api-go-client,skyeunknown084/gmail-api,jfcote87/api2,stephenwithav/google-api-go-client