4.x - App Factory & ServerRequest Creator Auto-Detection

[l0gicgate]

This PR introduces AppFactory which enables you to create an app without having to create ResponseFactory and also gives the ability to the end user to run the app without having to manually create a ServerRequest given that they have installed one of the supported PSR-7 implementations / ServerRequest creator combos.

In order for auto-detection to work you need to install one of the following supported combos:

• Slim-Psr7 - Install using composer require slim/psr7:dev-master
• Nyholm/psr7 & Nyholm/psr7-server - Install using composer require nyholm/psr-7 nyholm/psr7-server
• Guzzle/psr7 & http-interop/http-factory-guzzle - composer require guzzlehttp/psr7 http-interop/http-factory-guzzle
• zend-diactoros - Install using zendframework/zend-diactoros

Hello World using PSR-7 & ServerRequest creator auto-detection

<?php
require 'vendor/autoload.php';

use Slim\AppFactory;

/**
 * The AppFactory::create() method takes 5 optional parameters
 * 
 * @param ResponseFactoryInterface Any implementation of a ResponseFactory
 * @param ContainerInterface|null Any implementation of a Container
 * @param CallableResolverInterface|null Any implementation of a CallableResolver
 * @param RouteCollectorInterface|null Any implementation of a RouteCollector
 * @param RouteResolverInterface|null Any implementation of a RouteResolverInterface
 */
$app = AppFactory::create();
$app->get('/hello/{name}', function ($request, $response, $args) {
    $response->getBody()->write("Hello, " . $args['name']);
    return $response;
});

$app->run();

Closes #2621

[coveralls]

Coverage increased (+0.9%) to 99.457% when pulling fd2aef9 on l0gicgate:4.x-AppFactory into 36d6786 on slimphp:4.x.