Teleport passenger/vehicle with player

Will teleport entities with a player for vehicle/passenger
Implemented LeadKnot. Adds BUKKIT-4570
MonsterEggSpawn Event

Get the itemstack used to spawn an entity
Call EntityDeathEvent when Creeper explodes. Fixes BUKKIT-1812
Return the Horse for HorseInventory.getHolder(). Fixes BUKKIT-4466
restrict spawn in memory to view distance
Improvements to Player lookup - Resolves BUKKIT-4608

Many sections of code does player lookups by iterating every online player
and doing name comparisons. This is very ineffecient and can reduce performance
on servers with high player count.

By using a map based approach for most player lookups, player lookup
performance should scale better with increased amount of players.

This commit also corrects a loose lookup on adding and removing ops by
using getPlayerExact instead, as adding and removing ops is not a loose
match operation.