Add polls

• store questions as activities
• allow users to vote (by creating note activity)
• make sure federalization is working
• options in oneOf or anyOf need to be Note objects instead of bare strings
• fix bug after creating a status with a poll extra empty status appear
• attributedTo is referent to the object instead of the actor which created it.
• endTime needs to be in the AS2 date format (ISO8601) instead of the number of seconds.
• as:Public is not present in cc field, even though I posted it as a public poll.