Add Model class with first tests

Add GameState enum.
Add position setters to Player.
Add GameObjectFactory

Make GameObject class public.
Add empty TankFactory.
Add some empty tests for Model.
Add player factory
Add method for model update
Create player in one place only
Add counters to model

current level
points
enemies to beat
player's lifes
Merge branch 'feature/Model' into develop
Interface for Observer design pattern
Add Model class with empty tests
Add easymock library to project
Add empty GameCore class
Fix interface for Observer design pattern