1.0.0 Roadmap #3

This is largely based on the featureset needed by google-cloud-python and the features provided by google-auth-library-nodejs.

Required features:

• JWT signing and verification using PEM keys. (Add google.auth.crypt #6, Add google.auth.jwt #7)
• JWT credentials. (Add JWT credentials #21)
• Compute Engine credentials. (Add compute engine metadata client #11, Add compute engine credentials #22)
• App Engine credentials. (#
• Service Account (JWT Grant for OAuth 2.0) credentials. (Add google.oauth2._client #13, Add service account credentials #25)
• OAuth 2.0 Access Token credentials (no flows, credentials only for ADC). (Add google.oauth2._client #13, Add oauth2 credentials #24)
• Implementation of application default credentials, along with the ability to determine the project ID. (Implement application default credentials #32)
• Well-defined transport interface, with ability for users to build their own. (Add http.client transport #10)
• urllib3 support. (Add urllib3 transport #14)
• Ability to convert oauth2client credentials. (Implement App Engine credentials #36)

Nice to have:

• requests support. (Implement Requests transport #37)
• helper to verify Google-signed ID tokens oauth2client's implementation. (Implement google.oauth2.verify_id_token #39)

oauth2client features explicitly not in scope for 1.0.0:

• OAuth 2.0 flows.
• Storage. (Discussion at [Discussion] Storage and locking #33)
• Devshell credentials (seems to only be used by the gcloud cli).
• Django, Flask, and Webapp2 helpers.
• httplib2 support. (Temporary support in Add private httplib2 transport #34)