add config class and schedule

spec/extensions/active_job_spec.rb

@@ -0,0 +1,16 @@
+require "spec_helper"
+
+class TestJob < ActiveJob::Base
+  def perform;end
+end
+
+TestJob.extend(Periodicity::Extensions::ActiveJob)
+
+describe Periodicity::Extensions::ActiveJob do
+  describe "#perform_every" do
+    it "should add job and period to schedule" do
+      TestJob.perform_every(1.second)
+      expect(Periodicity::Config.instance.schedule).to_not be_empty
+    end
+  end
+end