Challenge another player

[marein]

Once #6 and #14 are done, players could challenge each other directly by sending a challenge. The opponent can accept or reject the challenge. The proponent can withdraw the challenge if not yet accepted or reject. When a challenge is accepted, the players are paired.

Additional requirements:

• An anonymous player may not challenge other players if the challenged player denied that in their settings.
• Players are notified about the challenge either via push notification or via a notification center (+ push notification). The disadvantage of the first one is that they've to be online.

Technical notes:

• This makes Game::open() obsolete. A challenge is something that should be explicit in the domain. To allow the Game::open() functionality, a challenge can be "open", so anybody could join.