Prep for pr

This PR includes a few features we have been using internally for a while including:

* Ability to check groups that are composites of other groups
```ruby
@rollout.define_group(:a) { |user| user.a }
@rollout.define_group(:b) { |user| user.b }
@rollout.define_group(:c) { |user| user.c }

@rollout.activate_group("some_feature", "a&b!c")

@rollout.active?("some_feature", user) # true if <#User a:true b:true c:false>
```
* Add salt option to calculate users that get a feature when based on a percentage. (trobrock/rollout@120e4736056c164f21b1524d5d63634791bcb22b)
* Add the ability to enable a feature for a percentage of a group

/cc @sleparc He's the one that did all the work