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* added 'current_happenings' template tag to display events that are
happening now.
* created a template partial called happening_events.html for the new tag.
It resembles upcoming_events.html.
* added .is_happening(now) function in models, so that you can call
event.is_happening(now) to see if the event is happening 'now' (where
'now' is an optional datetime, and by default is right now)
* added tests for the new 'current_happenings' tag, but I was getting
a strange invocation error when I tried to put the tests into their
own file, so I stuck the tests in with upcoming_events tests.