Misc PDO improvements

This PR brings a few misc improvements:

- the inline comments all mention SQLite, giving the false impression that this class is designed to interact with an SQLite DB only
- to make that clearer, I renamed the demo file to `demo_pdo_for_sqlite.php` and added an example file to interact with a MySQL server, named `demo_pdo_for_mysql.php`
- Also, the original example file for SQLite was broken: it was missing the inclusion of file `ez_sql_pdo.php` itself
- I fixed the `sysdate()` function to actually work on a MySQL server, which I thought is probably the widest use for this class