Process incoming json documents #89

Inspired by #74, it would be nice if this library can also help with incoming requests. There is already tickets for validating incoming requests (#58, #57). This issue is about also converting the document into objects you can pass to the rest of your application. E.g. a RelationshipObject. Still to define a bit more what exactly we can help implementations with.

Some first ideas:

• process incoming relationships into something like ResourceIdentifierObjects
• implement PSR-7, especially ServerRequestInterface->getParsedBody()
• helping with the difference between requests updating a resource and those updating a relationship