Allow multiple ad_slots for each ad channel.

I was in the need to be able to allow multiple ad_slots for each channel as it appears the module only creates one ad_slot per channel for each ad cycle. I needed to be able to set a configurable amount of ads that will then be displayed randomly in the ad_group. This was needed because there will be a large number and large variety of ads being displayed on the site I am working on. I went ahead and created a simple patch that allows an administrative option for setting the amount of ad_slots that will be created for each ad_channel.

I am not sure if this is something you would be interested in adding to ad_auctions but I wanted to provide it here in case anyone else needed something similar. I have only briefly tested its functionality and it seems to be working correctly.

Another note: the patch is rather large because my editor removed some space from the end of lines automatically. The actually amount of code that was added was fairly minimal (10 lines or less).