google-api-go-client: Add a gmail API example.
google-api-go-client: Add a gmail API example.

R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/109290044

maskofG/google-api-go-client,tmaiaroto/google-api-go-client,Shino1024/google-api-go-client,aquarat/google-api-go-client,maskofG/google-api-go-client,Shino1024/google-api-go-client,abujehad139/google-api-go-client,aquarat/google-api-go-client,tmaiaroto/google-api-go-client,abujehad139/google-api-go-client