Common Database interface

I grown frustrated to not have my IDE autocomple the database objects methods so I extracted the mysql.class.php one into a nice interface and made the abstract DoliDB class I had prepared implement it.

This revealed a bunch of missing implementations in the other drivers I tried to fix and document as much as I could.

Next step will be to factor common methods into the abstract class so we don't have duplicated code to maintain.

Please comment, merge, use and enjoy ;)