Add Codeception bootstrap files
Changes list:
+ helper functions in `/src/Wel/Mediafire/Utils/helpers.php`.
+ implement `Mediafire::getSessionToken()`.
+ implement `User::getSessionToken()`.
+ implement `User::getTos()`.
+ fix token expired error.
+ add my own *response result codes* to `\Wel\Mediafire\Messages\Response`.
+ add `\Wel\Mediafire\Responses\TermOfService` Value Object.
+ change example file `/examples/get_token.php` to /examples/user_api.php`.
+ add `\Wel\Mediafire\Exceptions\InvalidMethodException` exception.