Google-auth

- Added ``jwt.OnDemandCredentials``. ([142](https://github.com/googleapis/google-auth-library-python/pull/142))
- Added new public property ``id_token`` to ``oauth2.credentials.Credentials``. ([150](https://github.com/googleapis/google-auth-library-python/pull/150))
- Added the ability to set the address used to communicate with the Compute Engine metadata server via the ``GCE_METADATA_ROOT`` and ``GCE_METADATA_IP`` environment variables. ([148](https://github.com/googleapis/google-auth-library-python/pull/148))
- Changed the way cloud project IDs are ascertained from the Google Cloud SDK. ([147](https://github.com/googleapis/google-auth-library-python/pull/147))
- Modified expiration logic to add a 5 minute clock skew accommodation. ([145](https://github.com/googleapis/google-auth-library-python/pull/145))

- Added ``service_account.Credentials.with_claims``. ([140](https://github.com/googleapis/google-auth-library-python/pull/140))
- Moved ``google.auth.oauthlib`` and ``google.auth.flow`` to a new separate package ``google_auth_oauthlib``. ([137](https://github.com/googleapis/google-auth-library-python/pull/137), [#139](https://github.com/googleapis/google-auth-library-python/pull/139), [#135](https://github.com/googleapis/google-auth-library-python/pull/135), [#126](https://github.com/googleapis/google-auth-library-python/pull/126))
- Added ``InstalledAppFlow`` to ``google_auth_oauthlib``. ([128](https://github.com/googleapis/google-auth-library-python/pull/128))
- Fixed some packaging and documentation issues. ([131](https://github.com/googleapis/google-auth-library-python/pull/131))
- Added a helpful error message when importing optional dependencies. ([125](https://github.com/googleapis/google-auth-library-python/pull/125))
- Made all properties required to reconstruct ``google.oauth2.credentials.Credentials`` public. ([124](https://github.com/googleapis/google-auth-library-python/pull/124))
- Added official Python 3.6 support. ([102](https://github.com/googleapis/google-auth-library-python/pull/102))
- Added ``jwt.Credentials.from_signing_credentials`` and removed ``service_account.Credentials.to_jwt_credentials``. ([120](https://github.com/googleapis/google-auth-library-python/pull/120))

- Removed one-time token behavior from ``jwt.Credentials``, audience claim is now required and fixed. ([117](https://github.com/googleapis/google-auth-library-python/pull/117))
- ``crypt.Signer`` and ``crypt.Verifier`` are now abstract base classes. The concrete implementations have been renamed to ``crypt.RSASigner`` and ``crypt.RSAVerifier``. ``app_engine.Signer`` and ``iam.Signer`` now inherit from ``crypt.Signer``. ([115](https://github.com/googleapis/google-auth-library-python/pull/115))
- ``transport.grpc`` now correctly calls ``Credentials.before_request``. ([116](https://github.com/googleapis/google-auth-library-python/pull/116))

- Added ``google.auth.iam.Signer``. ([108](https://github.com/googleapis/google-auth-library-python/pull/108))
- Fixed issue where ``google.auth.app_engine.Signer`` erroneously returns a tuple from ``sign()``. ([109](https://github.com/googleapis/google-auth-library-python/pull/109))
- Added public property ``google.auth.credentials.Signing.signer``. ([110](https://github.com/googleapis/google-auth-library-python/pull/110))

- Added experimental integration with ``requests-oauthlib`` in ``google.oauth2.oauthlib`` and ``google.oauth2.flow``. ([100](https://github.com/googleapis/google-auth-library-python/pull/100), [#105](https://github.com/googleapis/google-auth-library-python/pull/105), [#106](https://github.com/googleapis/google-auth-library-python/pull/106))
- Fixed typo in ``google_auth_httplib2``'s README. ([105](https://github.com/googleapis/google-auth-library-python/pull/105))

- Added ``app_engine.Signer``. ([97](https://github.com/googleapis/google-auth-library-python/pull/97))
- Added ``crypt.Signer.from_service_account_file``. ([95](https://github.com/googleapis/google-auth-library-python/pull/95))
- Fixed error handling in the oauth2 client. ([96](https://github.com/googleapis/google-auth-library-python/pull/96))
- Fixed the App Engine system tests.