Feature/user object or

Resolves https://github.com/FetLife/rollout/issues/38

- This is BC-compatible
- But you can also use id's rather than some type of `User` object if you want:
```ruby
rollout = Rollout.new Redis.new
rollout.define_group(:fives_only) { |user_id| user_id == 5 }
rollout.activate_group(:chat, :fivesonly)
rollout.active?(:chat, 5) 
#-> true
```