make using sets for users and groups optional

[burin]

PR to address #103

This PR makes using sets optional:

Rollout.new(Redis.new, use_sets: true) will use the old behavior with sets

Rollout.new(Redis.new) will use the "new" behavior with arrays. This would be the new default. If you prefer to use set as default, I can open a new PR and make the "new" behavior optional.

require 'redis'
rollout_client = Rollout.new(Redis.new)
iterations = 100
rollout_client.activate(:test_feature)
raw_users = []
100_000.times {|num| raw_users << num }

rollout_client_sets = Rollout.new(Redis.new, use_sets: true)
rollout_client_sets.activate_users(:test_feature, raw_users)

Benchmark.bm do |bm|
  bm.report do
    iterations.times do
      rollout_client.active?(:test_feature, '123')
    end
  end

  bm.report do
    iterations.times do
      rollout_client_sets.active?(:test_feature, '123')    end
  end

end

### 3.650000   0.200000   3.850000 (  3.897240)
### 13.150000   0.250000  13.400000 ( 13.426378)

[burin]

@reneklacan any feedback on this PR? In particular the default behavior: should it use sets by default or arrays? (this PR changes default behavior)

[reneklacan]

@burin sorry, I missed PR submission, it looks all good 👍

[burin]

thanks @reneklacan for the quick turnaround