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
remove as_source._creation... helper methods
change CreationFailure to CreationResult unifying return value from create methods
Merge pull request #15 from kylewm/publish-reply-errors2

publish: improve error reporting for comments, likes, and shares
move requests mocking helpers here from bridgy

cc @notenoughneon. you should be able to use self.expect_requests_post() in a unit test now to test f0f0de795e319f4a863305a3ecc1155528274c90.
facebook publish: use USERID_POSTID format with likes endpoint. fixes #229
Added ability to get tweets by list timeline using arbitrary group_ids, failing test.
Fixed+added tests for twitter list fetching

Split tests in to explicit and implicit username tests, as different code paths are taken and different requests made.
Added docstring paragraph covering twitter lists
Merge pull request #17 from barnabywalters/master

Added ability to get tweets by list timeline
update oauth_dropins
update oauth_dropins
add unit test for twitter create with photo

for snarfed/bridgy#85
update webutil