Make *Error implement Unwrap (#66)

diff --git a/xattr.go b/xattr.go
index f982da3..8b2b5fe 100644
--- a/xattr.go
+++ b/xattr.go
@@ -29,6 +29,8 @@
 	Err  error
 }
 
+func (e *Error) Unwrap() error { return e.Err }
+
 func (e *Error) Error() (errstr string) {
 	if e.Op != "" {
 		errstr += e.Op