Leaderboard

* Allow options to be passed down to the connection pool.

* Allow a Redis connection to be passed in the Leaderboard initializer using the `redis_connection` option.

* Similar fix to one found in the [Ruby leaderboard](https://github.com/agoragames/leaderboard) library. Fixes TieRankingLeaderboard doesn't rank if the score is 0.
* Allow member data to be set in the `change_score_for(...)` method.
* Add `include_missing` option in leaderboard request options to change
whether or not to include missing members in the result.

* Add `global_member_data` option that allows multiple leaderboards to share the same set of member_data. [51](https://github.com/agoragames/leaderboard/pull/51) for original pull request from the Ruby leaderboard library.
* Add `top` helper method. [50](https://github.com/agoragames/leaderboard/pull/50) for the original pull request from the Ruby leaderboard library.

* Add support for `change_score_for(...)` in the `TieRankingLeaderboard` class.

* Add Python3 version check to accommodate 2to3 change of itertools.izip_longest [30](https://github.com/agoragames/leaderboard-python/pull/30)