preserve habit-style max repeaters and include in to_string()

Here's a fix for the to_string() method for habit-style max repeater values.

I wasn't sure how to associate that with the existing recurrence attribute or if a separate DateTime::Event::Recurrence object should be created.  The existing attribute is private, so I did not add it to the documentation.  If you know what you'd like to see done there, let me know and I'd be happy to work on a patch.

wu