google-api-go-client: add Application Default Credential example
google-api-go-client: add Application Default Credential example

I've copied the example that Chris wrote here:
https://developers.google.com/identity/protocols/application-default-credentials#callinggo

Fixes #89

Change-Id: Ib565a0280f6b42151d7876e1a50a0d5d416e2e90
Reviewed-on: https://code-review.googlesource.com/3263
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

80nashi/google-api-go-client,stephenwithav/google-api-go-client,google/google-api-go-client,googleapis/google-api-go-client,webhost/google-api-go-client,digideskio/google-api-go-client,googleapis/google-api-go-client,google/google-api-go-client,likaiwalkman/google-api-go-client