Ability to specify display chance as a fraction of 100%

Advertisement should have the capability to control the amount an ad is displayed as a fraction of 100%.

For instance, if you would like one ad to have 20% of the views and another to have 80%. Or if you have four ads and you'd like them displayed at 50%, 25%, 17%, and 8% of the time.

The old banner module implemented this via the 'chance' field, where the odds that an item were shown was (ad chance)/(sum of all ad chances).