Add exercies of chapter 02.

/cc @fabioyamate, @laurocaetano
The expression was:

```haskell
10 `div` length xs
```

This is bounded to the following:

```haskell
(div 10 (length xs))
```

Why?

Because of infix precedence. `div` has `infixl 7` which means it is bounded to
the left side of the expression with value 7. Every function that's used by
juxtaposing the function and argument (a.k.a the usual haskell notation) can be
thought of "having" a precedence value of 10, explaining why `length xs` is
applied before `div`.

Another example:

```haskell
2 + 3 * 4
```

Forget the math rules, just look at the infix precedence levels of each function.
`(+)` has `infixl 6` and `(*)` has `infixl 7`.

Then the expression is bounded as:

```haskell
(2 + (3 * 4))
```

Got it?

/cc @fabioyamate @laurocaetano