Add support for PSR 7 responses and requests via guzzle
This should be backwards compatible and not cause issue with existing controller's $this->request which is an instance of \sb\Request.
Instead the \sb\Request will have a new toPsr7() method that will convert the request to a psr7 compliant request object for better interoperability with other libraries like the league oauth2 library etc
The gateway should also render any psr7 compliant response returned from a controller method. Again this should be totally backwards compatible.
Should use guzzle in order to not have to reinvent the wheel.