pet system rework

[jasongdove]

TODO:

• rework pet loading on login (current pet instead of all pets)
• warlock dismiss and summon should use same pet
• warlock cycling pets should use pets that already exist, or create a new pet
• hunter should be able to tame multiple pets
• rework hunter stable opcode handling
  • fix WorldSession::HandleSetPetSlot
  • fix WorldSession::HandleStableChangeSlotCallback
  • fix WorldSession::SendStablePet
• add playercreateinfo_cast_spell system for hunter default pets, gnome auras

BUGS:

• bugged characters (with multiple pets) show multiple times in char enum, because they all have slot zero
• taming a new pet as a hunter overwrites the current pet
• hunter call pet on action bar doesn't open list