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.

spec/producer/core/action_spec.rb

@@ -0,0 +1,21 @@
+require 'spec_helper'
+
+module Producer::Core
+  describe Action do
+    let(:env)         { double 'env' }
+    let(:arguments)   { [:some, :arguments] }
+    subject(:action)  { Action.new(env, *arguments) }
+
+    describe '#env' do
+      it 'returns the assigned env' do
+        expect(action.env).to eq env
+      end
+    end
+
+    describe '#arguments' do
+      it 'returns the assigned arguments' do
+        expect(action.arguments).to eq arguments
+      end
+    end
+  end
+end