Change MsgBase.get_user_votes() to MsgBase.how_user_voted().

New MsgBase header propeties:
- total_votes (total number of votes this msg or poll has received)
- tally[], an array of 16 numbers indicating the total votes for each poll answer
  in the case of a normal message, tally[0] is the same as upvotes and tally[1]
  is the same as downvotes.
This should complete what echicken needs to display polls and voting results
in his web UI.