Add Google App Engine Service Account Support

[anthmgoogle]

Add support for the GAE built-in service account, aka. App Identity. Also, add support to Application Default Credentials for detecting this environment and automatically using this identity.

[ajkannan]

From the public documentation, it looks like the Google Application Default Credentials can detect the App Engine service account. Does that mean that this issue is now fixed? cc/ @aozarov

[aozarov]

And if it was fixed how does it behave in AE dev appserver?

[anthmgoogle]

This was implemented in this library in February 2015. In the Dev App Server it picks up the local application default credentials for use as a proxy for the app identity.