Finish luhn validation exercise

I find it helpful to break out little functions for conditions:

``` haskell
divisibleBy :: Int -> Int -> Bool
divisibleBy denominator numerator = numerator `mod` denominator == 0
```

You can then simplify `validate`:

``` haskell
validate :: Int -> Bool
validate n = divisibleBy 10 $ sumDigits $ doubleEveryOther $ toDigits n
```

Once `validate` is pure composition, you can go point-free:

``` haskell
validate :: Int -> Bool
validate = divisibleBy 10 . sumDigits . doubleEveryOther . toDigits
```