Register the serializer each format needs, so far they're all :json

This also contains a refactoring of how registering a format works, and
JsonApi has been renamed to JsonAPI.
YARD updates and fixes
Support for different output serializations

Instead of returning a raw hash/array from Yaks.serialize, include a
last processing step to serialize to string. This way Yaks can
distinguish between the "syntax tree" representation (for JSON: hashes
and arrays) and the final string form.

Still needed: tests, ability to override serializer, before/after/around hooks.