Add support for tie handling in leaderboards.

* TieRankingLeaderboard: Members with the same score are given
  the same rank.
* CompetitionRankingLeaderboard: Members with the same score will
  be given the same rank. A gap is left in the ranking numbers
  depending on where the next differing score falls.
Merge pull request #13 from agoragames/ties

Add support for tie handling in leaderboards.