A new RequestValidator `fill_id_token` has been introduced to replace `get_id_token`. It aims to have the bare minimum amount of fields to complete a full OIDC id_token support. `get_id_token` is still valid but optional, and if it is implemented, `fill_id_token` will not be called. The current `fill_id_token` came with full support of `aud`, `iat`, `nonce`, `at_hash` and `c_hash`. More could come in the future e.g. `auth_time`, ...

I fixed graphviz missing output to web responses (see image of https://github.com/oauthlib/oauthlib/pull/639), and I have added a fixed rank (`rank=same`) when functions are achieving an identical goal. E.g. `validate_client_id`, `validate_user`, `validate_bearer_token` are unique for each flows, or, e.g. `confirm_redirect_uri`, `validate_redirect_uri` together, and so on.

![graphviz-0cc58e8637b94d7402eda45a1fef6e68889bd8e1](https://user-images.githubusercontent.com/820496/50830407-042ad600-1348-11e9-936a-03d07f42494f.png)