Implement basic task actions API:

* Add Action base class;
* Implement Task::DSL.define_action(keyword, klass);
* Add Task::DSL#actions accessor;
* Apply task DSL actions when Task instances are evaluated.

lib/producer/core/action.rb

@@ -0,0 +1,12 @@
+module Producer
+  module Core
+    class Action
+      attr_accessor :env, :arguments
+
+      def initialize(env, *args)
+        @env        = env
+        @arguments  = args
+      end
+    end
+  end
+end