microformats2; ignore strings fields that we expect objects for (e.g. author)

fixes snarfed/bridgy#247
update oauth-dropins
publish: improve error reporting for comments, likes, and shares

- change create methods to return a tuple containing the result and
  a descriptive "failure" object (instead of throwing
  NotImplementedError)
- give informative error messages when publishing comments, likes,
  or shares without a target url for the given silo
- added unit tests for the more detailed error reporting for facebook
  and twitter publishing