Add Players enum class with special values

[micahstairs]

I'd love a Players class with constants like ALL, OPPONENTS, OTHERS, etc.

It would allow us to simplify code like:

$cards = [];
foreach (self::getPlayerIds() as $playerId) {
  $cards = array_merge($cards, self::getCards(Locations::SCORE, $playerId));
}

to

$cards = self::getCards(Locations::SCORE, Players::ALL);

It would also help us clean up code like:

if ($owner == -2) { // any player
            $owner_condition = "owner != 0 AND";
        } else if ($owner == -3) { // any opponent
            $opponent_ids = self::getActiveOpponentIds(self::getActivePlayerId());
            $owner_condition = self::format("owner IN ({owners}) AND", array('owners' => join(',', $opponent_ids)));
        } else if ($owner == -4) { // any other player
            $owner_condition = self::format("owner != 0 AND owner != {player_id} AND", array('player_id' => self::getActivePlayerId()));
        } else {
            $owner_condition = self::format("owner = {owner} AND", array('owner' => $owner));
        }