#26 — retrieveSingleFeed fails for certain feeds without guid or link

I have come across some feeds that w3c marks as valid but have no guid or link.
This fails when _retrieveSingleFeed in utility.py generates the id.
T checks
I did a quick patch for my site but my knowledge of rss is limited (it is the second try/except and the behavior of the except).
I just added another fallback in case link and guid are not present.

 93 for entry in parsed.entries:
 94 try:
 95 sig = md5.new(entry.id)
 96 except AttributeError:
 97 # Sometimes, rss providers send items without guid element.
 98 try:
 99 sig = md5.new(entry.link)
100 except AttributeError:
101 sig = md5.new(str(title_detail))
102 id = sig.hexdigest()