Modifying relationships

[shadowhand]

The current implementation doesn't handle modifying resources that have relations, because it submits related references in data.attributes instead of data.relationships.

For instance, assigning a post to an author should submit something like:

{data: {type: "posts", attributes: { ... }, relationships: {author: {data: {id: "1"}}}}

Refs #8

[henvo]

Hey @shadowhand thank you for your input!

I will have a look into this as soon as I have some time left.

But nonetheless pull requests are always welcome!