Test for DT::SpanSet and daylight savings change.

So, here's a test for all the daylight savings stuff.  I started with the example I gave before, but now it's got test for the switch off of daylight savings as well.

There are 64 failures with DT::SpanSet from svn, 0 with the patch posted earlier.

I did use DT::Event::Recurrence (the test skips if it's not installed); after looking at the guts of DT::Event::Recurrence, I couldn't figure out how to reproduce the stuff it does without grabbing about 2/3 of its code.  I know that there's a way of doing this that doesn't depend upon that module; I just don't know the what the recurrence callback should look like.