Common properties for each ZinaBB table

Every ZinaBB table contains 3 commen fields to enhance maintenance and reporting:
- sys_create
- sys_update
- sys_delete

sys_create
Contains the row's initial creation date and time.

sys_update
Contains the row's latest update date and time to register, determining when the row has been updated lately.

sys_delete
Contains the row's deletion date. So that the row has not to be deleted at first.

Therefor the standard Zend classes Zend_Db_Table_Abstract and Zend_Db_Table_Row_Abstract have to be extended to automatically fill and update these fields for every table interaction.