Allow to reverse the sorting order on search function

[twissell-]

For my particular use case, I need to sort events from newer to older. That is currently impossible as using dtstart sorts from older to new.

Luckily, adding an argument to the search function to reverse the order was quite easy.

I also added changes to the test suite. I couldn't run it as the test was skipped, but it should work. I tested the change within my project and worked just fine. It's not a big change.

[twissell-]

Closed with #433