Support for CPA commissions --> Revenue

[dczaretsky]

Currently the CPA campaigns are limited to fixed rates. However, when using conversion tracking, bucket values can vary. For example if you're tracking sales on an affilliate program.

When maintenance processes the tracked data, it rolls up the tracker variables of type "bucket value" and "number of items" into the table ox_data_summary_hourly under field "total_bucket_value" and "total_num_items".

It would be niced to have a new variable type called "commission" that would be rolled up into the total_revenue field, allowing us to support variable revenue based on a cost-per-sale model. Looking at the code base where bucket_values and num_items are rolled up, it doesn't look like it would take a lot of work.