Exercises 1, 3 added for chapter 11
Exercise 4 added for chapter 11
Bug in ch11e04 with Money(1, -25) fixed
Exercise 5 for chapter 11 added
Exercise 6 for chapter 11 added
Exercise 7 for chapter 11 added
Exercise 8 for chapter 11 added
Exercise 9 for chapter 11 added
Exercise 10 for chapter 11 added
Exercise 2 for chapter 11 added
Merge branch 'chapter11' into develop