Update docs to reference version 4.1

Bump version to 4.0.0

Run rustfmt

Rename set_*_url() to set_*_uri() (ramosbugs#128)

Re-add trailing parenthesis

Support RFC 7009 OAuth 2.0 Token Revocation (ramosbugs#122)

Rename reqwest-011 feature flag to reqwest

Add initial support for Device Code Flow as per RFC 8628 (ramosbugs#113)

* Add Client methods to set up the device authorization url and details
* Add Client methods to exchange for device codes, and to exchange codes
   for a token.
* Add an example that authorizes using Device Code Flow against Google.

Enable Serde for re-exported Url (ramosbugs#110)

Promote branch to 3.0.0