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.
Well, callable is "undeprecated" as of http://bugs.python.org/issue10518 so I guess callable() would be the way to do it?