Support automatic pluralization of types

Resolves https://github.com/Netflix/fast_jsonapi/issues/301

Enables support for pluralize_type in serializer definitions and relationship definitions, which pluralizes the type as expected.

Additional Note: In theory it's possible that we could try to infer whether relationship types should be pluralized by asking the relationship record's serializer about its pluralization state. But currently the code only interacts with the relationship record very lightly, and I was concerned about potentially missing performance targets for complicated records, so I didn't research this path very extensively.

This would help us move our serializers over to fast_jsonapi. Any update on this PR?

@sdhull I haven't heard back on any suggested changes from the maintainers. And the Travis builds break, for some reason.

You might want to note your interest in the original ticket, #301.

I wonder if you'd be willing to open this PR against https://github.com/fast-jsonapi/fast_jsonapi, which seems to me to be the way forward with this library, since the maintainers aren't responsive here. The travis failures seem completely unrelated to your changes, and I'd love to see this PR merged into that fork of this repo.