Refactor the Request objects

[acuthbert]

When I integrated the Http/HttpRequest object into the main Crown module it seemed silly to have two different Request objects in the one library. They both model the same thing so we should have a common ancestor in there object hierarchy. While we're at it, there is a PSR recommended interface for http request objects:

https://packagist.org/packages/psr/http-message

Interestingly they call it a message rather than a request. We should future proof ourselves by working with this interface if we want to stay php-fig compliant.

[acuthbert]

Not sure if this makes sense? The HttpRequest includes an addHeader() function for example which is used to set headers which makes no sense for an incoming request