Provide a hook_synchronize()

Here's a small patch doing just this. Modules implementing hook_synchronize() should use the following function signature:

/**
* hook_synchronize
*/
function my_module_synchronize($node, &$translation, $op) {

}

$translation needs to be referenced because its node_save() still happens in i18nsync. To synchronize custom fields you can do:

$fields = array('custom_field_1', 'custom_field_2', ...);
foreach ($fields as $field) {
  $translation->$field = $node->$field;
}