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
I think it's ready for the commits squash. I think the only other spec to add would be for the Competition Ranking type.