First exercise for the Third Chapter.
Solution for the second exercise for the third chapter.
Solution for the fourth exercise for the third chapter.
Fix type signatures.
Improve the answer for the fourth question.
Improving the answer.

See more info here http://www.cs.nott.ac.uk/~gmh/solutions.pdf
Merge pull request #3 from laurocaetano/ch-03

Solution for the third chapter.