whitelists and blacklists can be global

Until now, Feature.whitelist and .blacklist took a required first
argument -- the name of the feature:

    Arturo::Feature.whitelist(:foo) do |recipient|
      recipient.plan.has_foo?
    end

Now whitelists and blacklists can be global. The block takes the feature
as the first argument:

    Arturo::Feature.whitelist do |feature, recipient|
      recipient.plan.has?(feature.to_sym)
    end
Merge branch 'master' into james/global_whitelists
per-feature whitelists & blacklists implemented as global ones
Merge pull request #52 from jamesarosen/james/global_whitelists

whitelists and blacklists can be global