Leaderboard

* Added `remove_members_outside_rank` method to remove members from the leaderboard outside a given rank.

* Added `members_only` option for various leaderboard requests - HT: [Simon Zimmerman](https://github.com/simonz05)
* `leaders` call should return `[]` in case of an empty result set - HT: [Simon Zimmerman](https://github.com/simonz05)
* Initializer no longer deep copies options - HT: [Simon Zimmerman](https://github.com/simonz05)

* Fixed a data leak in `expire_leaderboard` and `expire_leaderboard_at` to also set expiration on the member data hash.

* Updated `remove_member` to also remove the optional member data for the member being removed.

* Added `rank_member_if` and `rank_member_if_in` methods that allow you to rank a member in the leaderboard based on execution of a function.
* Added `rank_members` and `rank_members_in` methods that allow you to rank multiple members in a leaderboard at once by passing in an array of members and scores.

* No longer cast scores to a floating point automatically. If requesting a score for an unknown member in the leaderboard, return `None`. Under the old behavior, a `None` score gets returned as 0.0. This is misleading as 0.0 is a valid score.
* Fixes a bug in `ranked_in_list` when requesting a list with an unknown member.