Make a constructor that takes a threading.Event object

[penguinland]

For this library to be most useful to other people, there should be a way of turning a threading.Event object into a multevents Event object. That way, you can start with several threading.Event objects created by other 3rd party code, and have a way of making an AnyEvent out of them.

However, it's unclear to me exactly how this should work. The original threading.Event object will be settable and clearable, which would set and clear the wrapper object as well. Should the wrapper object be settable/clearable, too? Part of me says "yes, then you can use the wrapper everywhere and ignore the original object." Part of me says "no, this is just another type of combo event, and we'd need to go out of our way to avoid infinite loops when the wrapper sets the inner object which sets the wrapper."

This should probably be done after #4 is finished. That issue will likely pave the way for this one.