Automatically setting endDate

[dnd]

I have a branch where I am implementing automatic setting of the endDate if the endDate is blank. In conjunction with issue #19 I was thinking I could add an option in there at the same time to prevent modification of the endDate. The default state of the option would be to modify the endDate thus retaining the current functionality.

Let me know if you would be interested in this, and I'll make the change and submit the pull request.