Added wishlist support

[lazyhummingbird]

I added functions for getting wishlists (saved lists) and their listings:

    @require_auth
    def get_wishlist(self, wishlist_id):
        """
        Get a wishlist's primary information
        """
        r = self._session.get(API_URL + "/wishlists/" + wishlist_id)
        r.raise_for_status()

        return r.json()

    @require_auth
    def get_wishlist_listings(self, wishlist_id, offset=0, limit=100):
        """
        Get a wishlist's listings
        """
        params = {
            'wishlist_id':str(wishlist_id),
            '_format': 'for_collaborator',
            '_offset': str(offset),
            '_limit': str(limit)
        }

        r = self._session.get(API_URL + "/wishlisted_listings", params=params)
        r.raise_for_status()

        return r.json()

Feel free to add/merge if desired. Just posting here to help in case others want it.

Thanks for a great library! Helped me plan a fantastic escape for my girl and I.