publish: discover rel=syndication copies of inReplyTo 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
- still TODO unit tests and error reporting
publish: added tests for expand_in_reply_to urls

Added unit tests to cover the functionality of expand_in_reply_to.
Tries a variety of post types, with syndication urls represented
a few different ways.

expand_in_reply_to_tests
 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
healthy mix of verb and target
bugfix: allow publish to more than one silo

This is described in issue #221
Merge pull request #231 from kylewm/more-than-one

bugfix: allow publish to more than one silo
publish: expand_target_urls polish and clean-up

based on suggestions from Ryan
Merge pull request #230 from kylewm/publish-ppd

publish: discover rel=syndication copies of inReplyTo & object urls
update unit tests to resolve code for #223 and #227