Expire, "attachment" element and database structure

- I added a hook for the "expire" script. In the addons I'm using this hook to remove old entries from facebook and twitter. (the changes in the addons will come in the next days)
- There is a new bbcode element called "attachment". It will replace the combination of several bbcode elements that now is used when a link is attached. This change will be done in the future.
- The new script to do database changes is now integrated into the update system and the installation process. **This isn't tested by now** - but the script itself runs fine. We should try a fresh installation before the new release to make sure that the script works like expected.