Changed some type(self)(start_offset=..., stop_offset=...) in Timespan.

The new syntax is:

    new(self, start_offset=..., stop_offset=...)

This is important because it allows Timespan subclasses to be make-new'd,
maintaining keyword arguments beyond start_offset and stop_offset.

Eventually, all uses of type(self)(...) should be changed to new(self, ...)
everywhere.