Added and corrected comments

id.go

@@ -12,10 +12,12 @@ type ID struct {
 	URI IRI `xml:"uri"`
 }
 
+// NewID creates a new ID. It returns a *ID.
 func NewID() *ID {
 	return &ID{URI: IRI(fmt.Sprint("urn:uuid:", uuid.New()))}
 }
 
+// Check checks the ID for incompatibilities with RFC4287. It returns an error.
 func (i *ID) Check() error {
 	if i.URI == "" {
 		return errors.New("uri element of id empty")