Make reservation number optional

The king of reservation code sent to the client, with or without a prefix, often confuses the clients, and take more time at the phone, for the personnel of the restaurant, in order to identify the code and the related reservation, for which the client is calling for

It is way better just to send the ID, which is a simple ascending number that easily is read and identified

To avoid to significantly hack the module, I just added this line

+447 $reservation_number = $reservation_id; // Avoid the ugly format of reservation number

but it will be much better if there's an option into the settings panel

admin/config/restaurant/reservation

That lets choose to switch between the 2 options:
- the (prefix)+MD5 code
- the simple ID