Create NewIcon and appropriate checks

icon.go

@@ -1,15 +1,26 @@
 package atomfeed
 
-import "errors"
+import (
+	"errors"
+	"fmt"
+)
 
 type Icon struct {
 	*CommonAttributes
 	URI URI `xml:"uri"`
 }
 
+func NewIcon(uri string) *Icon {
+	return &Icon{URI: URI(uri)}
+}
+
 func (i *Icon) Check() error {
 	if i.URI == "" {
 		return errors.New("uri element of icon empty")
+	} else {
+		if !isValidURI(i.URI) {
+			return fmt.Errorf("uri attribute %v of icon not correctly formatted", i.URI)
+		}
 	}
 
 	return nil