google-api-go-client: Add Fitness API example.
google-api-go-client: Add Fitness API example.

R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/171310043

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