Define top(number) helper method

I just wanted to add a shorthand for get top_10 of a specific leaderboard, but this cover a general way:

```ruby
leaderboard.top_10
leaderboard.top_5
```

I'll add the test cases and proper document if this method fit with the library
@czarneckid I  realized that in Tie Ranking type were missing the test case for `members_from_rank_range`. also do you think that this is ready for the commits squash?