Add link.targetSchema support

Hi @cyberdelia,

Here's my first take on support for targetSchema (issue #24)

I've used it with my API and it seems to be working nicely.

One big question: what should be returned for ```rel: instances```?

According to JSON Hyper-Schema:

> instances: This indicates the target resource that represents a collection of instances of a schema.

So I've set rel instances to return an array of targetSchema resources rather than returning targetSchema itself.

Let me know what you think.