manage exported file in "ExportedFile" model
dataset#updated_at field now represents the maximum updated time of the dataset and its attachments.

part of the work has been done in commit 85bc7d2229. whenever the attachment files are
updated, it will touch dataset#updated_at.
update triggers that expire exported Excel and csv files. issue #354
include download status of exported excel and csv in the XML of a dataset.
update schema and fixtures
update fixtures again. issue 354
update tests for triggers to expire exported data files. issue #354
Merge branch 'csv' into master

Conflicts:
	app/controllers/pages_controller.rb