Added duplication of variable definitions and a visitor to duplicate expressions (close #31)

...es/nperier/minichamo/language/definition/FunctionDef.java

...ennes/nperier/minichamo/language/definition/ValueDef.java

...es/nperier/minichamo/language/definition/VariableDef.java

...r/minichamo/language/expressions/ConditionExpression.java

...es/nperier/minichamo/language/expressions/Expression.java

...perier/minichamo/language/expressions/NameExpression.java

...nperier/minichamo/language/expressions/VarExpression.java

...guage/expressions/visitor/DuplicateExpressionVisitor.java

...language/expressions/visitor/SimpleExpressionVisitor.java