Add "page" param to ranking on apiv2

[JuanM4T]

For being able to get more than only the first 49 people on the rankings.

[tybug]

The proper way to do this is actually to use the cursor parameter, which is how the osu api handles pagination:

# using returned cursor which automatically increments to the next page
r = api.ranking("osu", RankingType.PERFORMANCE)
cursor = r.cursor
print(f"cursor page: {cursor.page}") # 2
print(r.ranking[0].global_rank)
r = api.ranking("osu", RankingType.PERFORMANCE, cursor=r.cursor)
print(r.ranking[0].global_rank)

# manually constructing a cursor
c = Cursor(page=3)
r = api.ranking("osu", RankingType.PERFORMANCE, cursor=c)
print(r.ranking[0].global_rank)

We need better documentation on this, but that's the intended usage.

[tybug]

I've fleshed out our readme more and added a section for pagination: https://github.com/circleguard/ossapi#pagination