Paginate relationships

[maggit]

I noticed that the method Follows and FollowedBy didn't support pagination, I added this support, so now you can paginate between people that you follow or people that follows you. It's very simple and uses the same structure than RecentMedia, and ResponsePagination now returns Cursor if this exists.

opt := &instagram.Parameters{Count: 20, Cursor: 1232324242}
users, next, err := client.Relationships.Follows("", opt)

opt := &instagram.Parameters{Count: 20, Cursor: 1232324242}
users, _, err := client.Relationships.FollowedBy("", opt)

[hamin]

@carbocation can we get this merged into master? All tests seem to be passing.

[hamin]

:+1: