Add exercies of chapter 02.

/cc @fabioyamate, @laurocaetano
hum, so me and @laurocaetano missed that and did the wrong fix.

so infix notation has less precedence.

AFAIK

```haskell
a `fn` b ~ div a b

-- but if a or b is an expression

a `fn` length b ~ fn a length b

-- or it adds implicit parens?

a `fn` length b ~ fn a (length b)
```