Provide hook allowing other modules to alter registration settings when an entity host is created or updated

The main idea is to provide two hook wich permit others modules to alter registration settings when an entity host is created or updated. It's an easy method wich permit to handle several use case as this issue Relative Date for Reminder Messages.

Attached a patch wich provides this 2 hooks (with drupal_alter) on registration_entity_insert() and registration_entity_update() implementations. These hook provide in the $context variable the entity host object.

These hooks are documented in registration.api.php. I also updated this doc with the hook available but not present in this file (hook_registration_status_alter).