Psr 7

** Do not merge, this ticket just exists for tracking purposes **

This branch represents a migration to PSR-7. PSR-7 addresses what our `RequestInterface`, `ResponseInterface` and `MessageInterface` also address.

Because of the benefits of standards-compliance and the potential ecosystem that may spawn around this, we'll start supporting this in a next major sabre/http version, 4.0.

Condition is that psr-http is finished.