Part 2 material published

Part 2.1 - Contract-Based Programming.md

Part 2.2 - Constructing Routines.md

Part 2.3 - Side-Effects and Exceptions.md

Part 2.4 - Encapsulation and integrity of a class.md

Part 2.5 - Constructing Groups of Classes.md

Part 2.6 - Creating a Class.md