CHANGED ENTIRE PROJECT STRUCTURE

-Added GTMEngine project
-Moved all model classes to the Engine project
-Changed all xml file loading to the xna importer
-Moved BXEL.dll to the Engine project folder (keeping a referencing to
it though)

-Added level and experience fields to the Hero class
-Added experience required per level (Check LevelUpExperiences.xml and
the new static field in the Hero class)