GamesHandler: Grace period after a user parts/quits

[FastLizard4]

Another feature a la lykos, allow users a grace period after they part or quit (for whatever reason) before the framework considers the player to have left the game.

Note: First version of Secret Hitler (and GamesHandler) is currently feature-frozen and held for final revisions, bug fixes, and code review before publishing. This issue is deferred until after this first version is code reviewed and published. See #22 and #20.