Update Part I: Introduction.md
Update Part II: Methods.md
Update Part II: Methods.md
Update Part III: Variables.md
Update and rename Part III: Strings.md to Part III: Variable methods.md
Update Part II: Methods.md
Update Part IV: Returning values.md
Update Part V: Accepting user input from the console.md
Update Part VI: Classes.md
Update Part VII: Exercises.md
Merge pull request #64 from BillsJ/master

Chapter II changes