birdnotes.py: Objects to represent bird note files

Generates the sightings in the order they were added. This can be implemented by returning an iterator for the ._sightingList attribute, which contains the sightings.

# - - -   B i r d F o r m . g e n S i g h t i n g s

    def genSightings(self):
        """Generate the sightings in self.
        """
        return  iter(self._sightingList)