Refactor populate_graph and generate_cross_feed_edges

[kuanb]

Per this comment (#4 (comment)):

    for i, row in wait_times_by_stop.iterrows():
        sid = str(row.stop_id)
        full_sid = nameify_stop_id(name, sid)

        # TODO: Join tables before hand to make
        #       this part go faster
        id_mask = (feed.stops.stop_id == sid)
        stop_data_head = feed.stops[id_mask].head(1)

This operation is performed twice during the final steps of graph generation. It's expensive and slow. Need to replace with a single merge operation whose results are passed to both methods.