Corrected Integer class definition

The Integer class now uses composition instead of inheritance since inheritance was mathematically incorrect.

Plus, .gitignore file was modified to suit better to my new development environnement.
Minor fixes

Fixed minor issues on Integer arithmetic operators overload caused by inheritance rework.
Merge feature 'changing-inheritance' into develop

Relation between Natural and Integer is now defined by composition instead of inheritance.

As a consequency, Integer is now a friend class of Natural.