Entities API

This PR represents the progress in adding all entities to Sponge. Ideally the end result should mirror the MC Source, Forge, and the Bukkit API in functionality.

__Players__
- [ ] OfflinePlayer/OnlinePlayer duality (#131)
- [ ] also in (#72)
- [ ] getHealth(), setHealth()
- [ ] getPosition(), setPosition()
- [ ] getMaxHealth(), setMaxHealth()
- [ ] getHunger(), setHunger(), getSaturation(), setSaturation()
- [ ] isUsingCraftingTable()
- [ ] getOpenInventory()

__Other__
- [ ] Non-mob entities (Items, Armour Stands, Paintings) (#94)
- [ ] hostile mobs
- [ ] friendly mobs
- [ ] villager trades?
- [ ] neutral mobs (Zombie Pigmen, Endermen, Wolves)
- [ ] ambient mobs (Bats) (#72)
- [ ] Entity naming (#45)
@Phroa this is a remnant from when OfflinePlayer did not extend Identifiable for whatever reason. Nice catch, although I don't want to fix it now.