db_view: views and triggers for mysql

"db_view" provides:
* An abstraction class and helpers for multi-tabled Mysql data.
(init, select, select_row, count, insert, update, delete)
* Atomic multi-table updates, inserts, deletes
eg: update items, month set items.price=month.price where items.id=month.id;
Also primitive non-atomic (=insecure) rollbacks.
* Functions, Triggers and Defaults per action
(select, update, insert, delete; before or after)
* Rich logging and debugging facilities.