Allow serializers to add links

[bwaidelich]

With this change Serializers can provide links like::

class PostSerializer implements SerializerInterface
{
    // ...

    public function getLinks($post) {
        return ['self' => '/posts/' . $post->id];
    }
}

Partly resolves #64

[Nitpick-CI]

Each class must be in a file by itself

[bwaidelich]

yeah well.. nit pick yourself ;)

[bwaidelich]

The Resource constructor expects those parameters

[bwaidelich]

FYI: This allows serializers to provide link relations (which I think is a crucial feature) partly resolving #64 – if this gets merged I'll gladly add another PR addressing the meta part

[bnchdrff]

I like. Hope this can get merged, as it's come in handy for a couple of my projects.

[tobyzerner]

Should a Resource instance's links take precedence over the serializer links?

[bwaidelich]

That's what I assumed (and what is covered by a test), but frankly I wasn't really sure about it.. And after thinking about it again it might be more flexible if a serializer can overrule resource links.
In the original patch by @bnchdrff it was also the other way around - should I swap it?

[tobyzerner]

Ah yes, didn't see the test and misread the order of array_merge arguments – the current behaviour is fine.