Merge pull request #3 from changebo/master

Added scripts for Chapter 5
Added table 6.1
Finished 6.3 Usage
Added Figure 6.12
Figure 6.13 and 6.14
Completed draft for Derek's part
Derek's section draft complete
Add Davor's content for Chapter 6
Reduce duplication in Gapminder exercises
Simplify the Gapminder example a bit more
Merge pull request #2 from cubranic/master

Chapter 6