Userpoints not updated when node/comment changes author

If a node or comment is updated with the author/owner changed to another user, userpoints are not correctly updated to reflect this change. There is some code in userpoints_nodeapi to handle the 'update' case, but it doesn't work because uid has already changed at that point. We need to capture and store the original uid at 'prepare' stage. Similar changes are made to hook_comment.

Patch attached, please review. Thanks.