Removing unused methods from Utils
Constructor for StudentModel instead of setters
Add toString methods to various classes

Most of the classes were using some kind of show() method. I'll remove
these as soon as possible; I'm trying to make small, incremental changes
at the moment.
Refactored StudentModel

(well, actually ModeloEstudante)
Change ModeloEstudante to StudentModel
Change Conceito to Concept
Change EstadoCognitivo to CognitiveState
Change EstiloAprendizagem to LearningStyle
Change ObjetivoAprendizagem to LearningObjective
Change ProcessoAprendizagem to LearningProcess
Remove Utils