Implement Elo rating

[marein]

Problem description
Jane and John want to know how they perform.

Possible solution
Create a new class named Player inside the context ConnectFour. Below there's an example structure of this class. The username is there as an example of how this issue supplements issue #14.

final class Player
{
    /**
     * @var string
     */
    private $id;

    /**
     * @var string
     */
    private $username;

    /**
     * @var int
     */
    private $eloRating;
}