Added support for PlaceHolderAPI and mVdWPlaceholderAPI.

PlaceHolderAPI:

%auctionhouse_active_listings% - count of active listings in the world of a player.
%auctionhouse_player_listings% - listings count for a player.
%auctionhouse_expired_listings% - expired listings count for a player.
%auctionhouse_sold_items% - sold items count for a player.
%auctionhouse_max_listings% - max number of listings permitted for a player.

mVdWPlaceHolderAPI:

{auctionhouse_active_listings} - count of active listings in the world of a player.
{auctionhouse_player_listings} - listings count for a player.
{auctionhouse_expired_listings} - expired listings count for a player.
{auctionhouse_sold_items} - sold items count for a player.
{auctionhouse_max_listings} - max number of listings permitted for a player.