Cleaning of WorldSession::HandlePlayerLogin to prepare for loot changes.

- Added additional comments.
- Added additional debugging.
Modified Player methods to handle more detailed checking of proper
looting in group environments.

- Added Player::isTappedByMeOrMyGroup method to check if a creatures has been
  tapped by either the Player or someone in the Player's group.
- Modified Player::isAllowedToLoot logic to properly check different
  group loot rules.