Re-work pagination and add tests.

Here's a first version. This version doesn't include `next` or `previous` in the metadata if a number for page can't be found. This is only really expected to happen when `next` or `previous` are null for the first and last page. An alternative would be include `next` and `previous` in the metadata and set them to -1 for the first and last pages. What do you think?

I also still need to write user facing docs but I'm waiting for that to be setup.
Does this belong nested in extractMeta, or should it be a private method of the serializer?  I don't have a preference either way, just curious to know what your reasoning is.