Allow "order by" clause to be defined in report criteria UI

This patch provides functionality described in http://civicrm.org/blogs/allenshaw/building-detailed-event-management-system-and-some-other-cool-stuff-civicrm under "Report sorting and group headers".

For the sake of example, this patch changes the "Constituent Report (Summary)" report to support a couple of additional sorting options, while keeping the report's current default sort_name order. There are 20-something other reports that could do with the same treatment, but I wasn't sure if I should presume to change them until this patch is reviewed. In the mean time, those reports continue to work as normal.

Known limitations: Existing column "sticky headers" and the new "order by" section headers don't play well together. Currently this code just disables sticky headers if "order by" sections are being used.

Questions and critiques are welcome, as always. I'm also happy to add documentation to the wiki when the status of this patch is clear.