Create isValidURN and isValidURI and use isValidURI everywhere where it is needed

category.go

@@ -27,6 +27,12 @@ func (c *Category) Check() error {
 		return errors.New("term attribute of category empty")
 	}
 
+	if c.Scheme != "" {
+		if !isValidURI(c.Scheme) {
+			return fmt.Errorf("scheme attribute of category %v not correctly formatted", c.Scheme)
+		}
+	}
+
 	if c.Content == nil {
 		return errors.New("no content element of category")
 	} else {