fix issue where duplicated matters had same date/time for different events

This solves issue #48 and a related issue where attachments list wouldn't show up for a matter that was repeated.

I did `git rebase master` so it should cleanly apply on top of the `master` branch. 

This replaces pull request https://github.com/codeforamerica/MuniciPal/pull/79 which I'll close now.