Add sort support by priority

Issue Summary

I thought this is all to evident, but looks like it was still missing from casetracker. We are using this module to help organize Drupalcon Szeged 2008, so we'd like to make it work well ;) Here is a simple patch we use to order our views of tasks by priority. I am hoping this gets into later versions, so we don't need to manage this patch if we need to update to later versions. This works well for us, and is a very simple patch.