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.
I just copied the block that handled "the other generator". I agree with "isinstance", but let's do that in a separate commit - and then fix both cases of course. I'll upload that soon.