Add answers for chapter 4 exercises
Add some lambda calculus exercises
No need to pattern match over empty list
Add another implementation of halve
Use print instead of putStrLn . show
Merge pull request #4 from fabioyamate/ch04

Chapter 4: Defining functions
Notes from chapter 5
First batch of exercises from chapter 5
Update notes for list comprehensions
Add answer for ex 7
Add answer for ex06
Add answer for exercise 8