Last digit calculation for ISBN-10 and ISBN-13

@igas, hi! I added correct calculation for last digit in sequence for ISBN-10 and ISBN-13. It still need a little refactoring, but it works.
Binary reverse is faster than list:
```elixir
iex(1)> :timer.tc(fn -> Enum.each(1..10_000, fn (_) -> String.reverse("1234567890") end) end)
{59552, :ok}
iex(2)> :timer.tc(fn -> Enum.each(1..10_000, fn (_) -> Enum.reverse(["1", "2", "3", "4", "5", "6", "7", "8", "9", "0"]) end) end)
{93649, :ok}
```

So before get list, revert binary.