Asset ID support (Issue #23)

Fixed issues with asset tracking and enabled checks on basic actions.

Tested against com_todo.

## To make this work
Every developer, to add asset tracking to his table, should:
- Add an asset_id field to the table
- Add the asset_id field as hidden to the form (xml or php)
- Add the field "rules" to his form (xml or php) with the correct section name