Turned MosaicSerializable into an interface and removed toString().
Refactored and finished implementation of deserialization methods.
Fixed equals() checking for illegal operand.
Let toNBTTag() return a CompoundTag for serialization.
Don't store item meta if it is null.
Added dummy implementation to convert SerializableItemMeta to ItemMeta.
Added toItemStack(). Does not imply ItemMeta, yet.