Use PSR's ContainerInterface.

* Tidying up some codes.

* Add more information badges.
* Use Slim's default container instead of PHP-DI.

* Fix #3: Get the request and response from the app container.

* Tidying up.

* Fix #1: `$request->getParsedBody()` always return null.

* Fix typo on `README.md`.

* FIX: Remove `declare(strict_types=1);` because it throws warning on…

… PHP 5.6.

This is the first release.

Tested using [codeception-zend-expressive-tests](https://github.com/Naktibalda/codeception-zend-expressive-tests)
and [codeception-rest-testcase](https://github.com/Naktibalda/codeception-rest-testcase) as a base.