Add support for PSR-7

[neomerx]

The lib uses \Neomerx\JsonApi\Contracts\Integration\CurrentRequestInterface which is really a small subset of PSR-7 (\Psr\Http\Message\ServerRequestInterface particularly). As many modern frameworks support PSR-7 this design solution adds extra efforts required to integrate the lib.

So it's time for #86 😄

[neomerx]

It's only a couple of methods are used from \Psr\Http\Message\ServerRequestInterface thus in frameworks which do not have native support for PSR-7 it could be reasonable to have light wrapper over native Request for performance reasons.
Therefore a Request class should be added to the lib with only those methods made abstract that are used in the lib. All other methods should throw an exception.