Issue 2: Fix deprecated notice

This includes a check for whether mysqli should be used.

This issue centres on the fact that PHP deprecated mysql functions in favour of mysqli, in version 5.5.

The `$wpdb` class does a check for the version, and sets the `use_mysqli` var accordingly. Therefore we can say that if `use_mysqli` is false, PHP is older than 5.5, and we shouldn't see a deprecated notice.

Further, if `use_mysqli` is true, then the `dbh` variable is set to a link that can be used by functions like `mysqli_get_server_info`.

Finally, I also included an `@` before each server_info function, so at the very least they return `''` instead of a notice.