Move Response class to React\Http\Message\Response and ServerRequest class to React\Http\Message\ServerRequest

[clue]

This changeset moves the Response class to React\Http\Message\Response and the ServerRequest class to React\Http\Message\ServerRequest. The Response class is used throughout our examples and user code, the ServerRequest class used to be internal only and is now exposed (testing client web applications).

// old
$response = new React\Http\Response(200, [], 'Hello!');

// new
$response = new React\Http\Message\Response(200, [], 'Hello!');

Builds on top of #361 and #368