Chapter 7: Higher-order functions

@carlosgaldino and @laurocaetano Finally I finished the chapter 7 exercises.

The exercise 7 took me some time because it wasn't work as I expected. But, once I did the evaluation steps it  was easy to fix it.

The exercises 8 and 9, not sure if it was the best solution.
Same thing here.

```haskell
*Main> dropWhile' even [1..5]
[1,3,5]
*Main> dropWhile even [1..5]
[1,2,3,4,5]
```

Your version in this case is a negative `select`, more like a `remove`, removing items which the predicate function returns `True`.