Add support for mysqli native PHP driver

This pull request add support for [native mysqli driver](http://php.net/manual/en/book.mysqli.php) in Tiny-Tiny-RSS

This is needed in future because mysql extension is **deprecated** as of PHP 5.5.0, and will be removed in the future.

It respects pgsql and mysql driver without affect to behaviour.

For use: define DB_TYPE as mysqli in config file:

    define(DB_TYPE, "mysqli");