add config class and schedule

spec/config_spec.rb

@@ -0,0 +1,10 @@
+require "spec_helper"
+
+describe Periodicity::Config do
+  describe "#initialize" do
+    it "should initialize schedule with an empty array" do
+      @config = Periodicity::Config.instance
+      expect(@config.schedule).to eql([])
+    end
+  end
+end