Fix OX-5959 Monthly tenancy campaigns have no Revenue/eCPM in statistics

> From https://developer.openx.org/jira/browse/OX-5959

The revenue and eCPM fields for Monthly Tenancy campaigns are always empty. That makes impossible to compare their performance to other campaigns and check if the tenancy deal is good or bad compared to them.

I've been asked by a customer to add the missing feature and they are willing to to contribute the patch back to the community.

I'm attaching the patch, kindly sponsored by http://www.admost.nl/