Entityform updates

This contains fixes for 3 Entityform related issues

Contributors can see unique and sensible column headers in Excel file downloads from form submissions.
https://www.pivotaltracker.com/story/show/78281502

CSV downloads include the date of the export
https://www.pivotaltracker.com/story/show/78920904

Form admin sees instance label in submissions table and downloads
https://www.pivotaltracker.com/story/show/78919630