Entities API

This PR represents the progress in adding all entities to Sponge.

__Players__
- [ ] OfflinePlayer/OnlinePlayer duality (#131)
- [ ] also in (#72)
- [ ] getHealth(), setHealth()
- [ ] getPosition(), setPosition()
- [ ] getMaxHealth(), setMaxHealth()
- [ ] getHunger(), setHunger(), getSaturation(), setSaturation()
- [ ] isUsingCraftingTable()
- [ ] getOpenInventory()