use <link> tag to extract the relative post url, fixes #37

The <id> tag does not necessarily contains the full URL, but also
relative URLs:

    <id>http://example.com/foo/bar.html</id>
    <id>/foo/bar.html</id>
    <id>foo/bar.html</id>

isso/migrate.py

@@ -30,7 +30,7 @@ comments = set([])
 
 def insert(db, thread, posts):
 
-    path = urlparse(thread.find('%sid' % ns).text).path
+    path = urlparse(thread.find('%slink' % ns).text).path
     remap = dict()
 
     if path not in db.threads: