Harmonize the revoke feature

I use a generic oauth2 resource owner, for my own oauth2 server.
I want to revoke the ``access_token`` when the user attempts to logout on the client side.

About the ``SinaWeiboResourceOwner``, the revoke method may not really revoke the token on ``master`` (not tested).
There is no documentation from the API for the revoke:
http://open.weibo.com/wiki/API%E6%96%87%E6%A1%A3_V2/en#OAuth_2.0_API_.28Version_2_APIs_only_support_OAuth_2.0_authorization.29
But the WS seems to need an ``access_token`` to revoke the access_token:
https://api.weibo.com/oauth2/revokeoauth2?access_token=example

My configuration with this PR:
```
    resource_owners:
        custom:
            type:                oauth2
            client_id:           %custom_oauth2_client_id%
            client_secret:       %custom_oauth2_client_secrect%
            access_token_url:    %custom_oauth2_host%/oauth/v2/token
            authorization_url:   %custom_oauth2_host%/oauth/v2/auth
            revoke_token_url:    %custom_oauth2_host%/oauth/v2/revoke
            infos_url:           %custom_oauth2_host%/oauth/v2/me

```