Revoke token via Java SDK

It is currently not possible to revoke a Google token using the Google Java SDK

See the documentation here:
https://developers.google.com/identity/protocols/OAuth2WebServer#tokenrevoke

There's no equivalent to the PHP `$client->revokeToken();` in Java. As far as I can tell, the only workaround is to create your own client for this API call.

This custom client then calls
```
curl -H "Content-type:application/x-www-form-urlencoded" \
https://accounts.google.com/o/oauth2/revoke?token={token}
```

As originally reported here:
https://github.com/googleapis/google-oauth-java-client/issues/20