Refactor task DSL usages:

Remove most of task evaluation code from Task class, rely on
Task::DSL.evaluate to get an evaluated task.

* Task: remove #evaluate, change constructor prototype to accept actions
  instead of a block, implement .evaluate(name, env &block);
* Implement Task::DSL.evaluate method;
* Recipe::DSL: remove tasks evaluation from#evaluate, modify #task to
  use Task.evaluate to return the new task to be rigstered.

spec/producer/core/task/dsl_spec.rb

@@ -12,6 +12,36 @@ module Producer::Core
       end
     end
 
+    describe '.evaluate' do
+      let(:name) { :some_task }
+
+      it 'builds a new DSL sandbox with given code' do
+        expect(Task::DSL).to receive(:new).once.with(&block).and_call_original
+        Task::DSL.evaluate(name, env, &block)
+      end
+
+      it 'evaluates the DSL sandbox code with given environment' do
+        dsl = double('dsl').as_null_object
+        allow(Task::DSL).to receive(:new) { dsl }
+        expect(dsl).to receive(:evaluate).with(env)
+        Task::DSL.evaluate(name, env, &block)
+      end
+
+      it 'builds a task with its name and registered actions' do
+        dsl = double('dsl').as_null_object
+        allow(Task::DSL).to receive(:new) { dsl }
+        allow(dsl).to receive(:actions) { [:some_action]}
+        expect(Task).to receive(:new).with(:some_task, [:some_action])
+        Task::DSL.evaluate(name, env, &block)
+      end
+
+      it 'returns the task' do
+        task = double('task')
+        allow(Task).to receive(:new) { task }
+        expect(Task::DSL.evaluate(name, env, &block)).to be task
+      end
+    end
+
     describe '.define_action' do
       it 'defines a new action keyword' do
         Task::DSL.define_action(:some_action, Object)