Create ComputeOAuth2Console class

[Scarygami]

For applications running on Compute engine there's a different way to receive access tokens for service accounts. Would be nice to create an according OAuth2Console class.

References:

https://developers.google.com/compute/docs/authentication
https://github.com/google/google-api-nodejs-client/blob/master/lib/auth/computeclient.js

[adam-singer]

adam-singer@3a69f04

Started to implement something. Have not yet had the time to test it out on compute engine.

[gmosx]

Please, please, please, implement this! I would love to be able to access Cloud Datastore with Dart.

[adam-singer]

updated that branch 8068722 need to test out tonight