Chapter 4 - Case study: interface design

[learnbyexample]

Chapter link: https://github.com/learnbyexample/ThinkRubyBuild/blob/master/chapters_md/case_study_interface_design.md

Main issue: couldn't install turtle in Ruby and had issues like mentioned here https://github.com/peterc/trtl/issues/5

Possible solution, use Exercise 3 (grid drawing) from Methods chapter as a base to teach concepts like Encapsulation, Generalization, Interface design etc