add a stats view to check for duplicate votes
add a filtering option to remove duplicate speakers

This removes all of a speaker's sessions except their highest voted one.
add some stats to show how many users have voted a given number of times
Merge pull request #175 from adrianbanks/more-voting-stats

This adds these stats:
  - duplicate votes made by the same user
  - a graph of the number of votes cast by a user vs the number of users who have made that many votes