Update 2012-10-19-revenue-report.md

Added screenshots and explanations of the new configurations in the revenue report. Deleted Mapping area as it is no longer needed by the Revenue Report. Existing mappings will "smart-default" in the new Revenue Report. Added copy of default behavior to use a churn time period of 30 days.
Update 2012-10-19-revenue-report.md

Updated churn time period with notes on 750 day period (over 2 years) as a limit for defining churn with the time period option. Provided link to mail feedback@kissmetrics.com as well if they have a use case for it.
Merge pull request #66 from chuckjliu/patch-5

Update 2012-10-19-revenue-report.md