API: posts_update.php should return last modification time

Hi,
The api function `posts_update.php` returns the time of the last created bookmark. The function is used to check, if an update of the local cache is necessary. For that it would be better to know the last modification time (instead of the creation time), otherwise the bookmarks metadata (descritption, status etc.) may become inconsistent.

The [delicious api](https://github.com/avos/delicious-api/blob/master/api/posts.md#v1postsupdate), seems to have the same problem: "Check to see when a user last posted an item".

In order to keep compatibility, I added an option: `posts_update.php?modified` will return the last modification time, while `post_update.php` behaves as it did before.