Support for generating refresh tokens

Due to 5d0fc1182f6d9826764cc43ade99bc5e5a81c711 in oauthlib,
there are now separate generators for access tokens and refresh
tokens.

Adding a hook to also specify refresh token generator.

Test cases are added.
(as I wrote above - callable() is "okey" again, so I'll don't upload any new commits to fix that, unless you insist)