publish: discover rel=syndication copies of inReplyTo & object urls

- when publishing a reply/repost/like, fetch the original post (if it's
  not on the webmention blacklist) and look for rel=syndication
  and u-syndication copies.
- expand the activitystreams object with the syndication urls and let
  the as.Source classes do the rest
- added unit tests for a variety of cases, with a mix of reply, share, like, rsvp
  1. ✓ simple u-syndication(s)
  2. ✓ simple rel-syndication(s)
  3. ✓ p-syndication h-cite
  4. ✓ primary h-entry is inside an h-feed
  5. ✓ failure to fetch inReplyTo url
  6. ✓ inReplyTo url has no microformats
  7. ✓ inReplyTo silo should not fetch anything

ref issue #223

@snarfed is it OK if we handle the improved error reporting (i.e., don't POSSE a reply as a comment if no reply post is found) in a separate PR and/or issue?
hmm, I think that would be fine. The only situation I can even imagine `children` being a problem is if the article has a nested `h-cite` without it being marked as a `p-comment`. Something like:
```html
<article class="h-entry">
  <span class="p-name">This is the text of the article I'm replying to</span>
  <a class="u-syndication" href="http://twitter.com/somebodyelse/1234">also on Twitter</a>
  Replies:
  <div class="h-cite">
    <span class="p-name">Hey that's a great point</span>
    <a class="u-syndication" href="http://twitter.com/a-third-person/5678">also on Twitter</a>
  </div>
</article>
```
But I hadn't thought about it that much... Mostly I was following the example from `post` up above, where it calls `attempt_single_item`.