Create end to end tests for common providers

[ib-lundgren]

These tests should be automated but could be excluded from the standard suite if there is no good way to hide real client ids and secrets in a test setup such as travis.

A good start would be for Google, Twitter and Facebook.

Ideally this should further help making new releases smoother. cc https://github.com/requests/requests-oauthlib/issues/157

[dasevilla]

Some of the OAuth flows require user interaction (i.e. sign in with your Google account and approve). Would this be for the OAuth 2.0 client credentials grant?

[JonathanHuot]

Closed as part of the 1.4.0 release https://github.com/requests/requests-oauthlib/pull/530 I'm suggesting to open a new MR for each "common providers" when working on it.