Create NewLogo and add checks

logo.go

@@ -1,15 +1,28 @@
 package atomfeed
 
-import "errors"
+import (
+	"errors"
+	"fmt"
+
+	"github.com/google/uuid"
+)
 
 type Logo struct {
 	*CommonAttributes
 	URI IRI `xml:"uri"`
 }
 
+func NewLogo() *Logo {
+	return &Logo{URI: IRI(fmt.Sprint("urn:uuid:", uuid.New()))}
+}
+
 func (l *Logo) Check() error {
 	if l.URI == "" {
 		return errors.New("uri element of logo empty")
+	} else {
+		if !isValidIRI(l.URI) {
+			return fmt.Errorf("uri element %v of logo not correctly formatted", l.URI)
+		}
 	}
 
 	return nil