Adds optional indented output for the Node.String() and Node.Bytes()

methods. Set the global `IndentPrefix` var to a tab or spaces to
enable indented output. The existing API has not been changed.

This addresses issue #7.

xmlx_test.go

@@ -74,6 +74,7 @@ func TestSave(t *testing.T) {
 		return
 	}
 
+	IndentPrefix = "\t"
 	if err := doc.SaveFile("test1.xml"); err != nil {
 		t.Errorf("SaveFile(): %s", err)
 		return