implement get_share() for Twitter to fetch a retweet
twitter: always use id_str instead of id
start to add source-specific text for likes and shares, e.g. 'retweeted this.'
add published datetime to twitter retweets
strip tags from retweets since we replace their content with 'retweeted this.'
fix test: add p-comment
implement instagram likes
instagram: reuse self.user_to_actor() for translating users_in_photo
facebook: reuse self.user_to_actor() for translating like users