implemented get_comments_url() and Atom Threading Extension

checks for RSS' comments element and stuff in [ATE](http://tools.ietf.org/html/rfc4685). Actually, it ignores some stuff in the RFC (*Processors […] MUST process each replies link independently of any others […] There is no implied relationship between the value of the "total"  element of an Atom entry and any individual or aggregate values of the "thr:count" attributes*), and my only defense is that it's atom.