OEC-486, rake tasks put CSVs to directories based on conventions of svn:OEC

https://jira.ets.berkeley.edu/jira/browse/OEC-486

New directory convention: "#{Settings.oec.export_directory}/data/#{term.year}-#{term.code}/raw/#{today}"
This looks a bit contorted to me, @johncrossman . As I understand it, the problem is that the CsvExport superclass expects to be able to set the "@export_dir" instance variable directly from a static string, but you need it to be more dynamic. Would you mind changing CsvExport to eliminate its invalid assumption? Or maybe just drop the subclassing, since it doesn't seem to gain you much?