Welcome to Pitchfork's best of 2008. We're going to count down our favorite 100 tracks of the year, 50 today and 50 tomorrow; the top albums come later this week. As we did in years past, we've extended the candidate pool beyond the confines of singledom-- basically any song released or covered in 2008, whether a single or not, was fair game for this list. Some songs that made a huge impact this y