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.

lib/producer/core/recipe/dsl.rb

@@ -22,7 +22,6 @@ module Producer
           else
             instance_eval &@block
           end
-          @tasks.each { |e| e.evaluate env }
           self
         end
 
@@ -41,7 +40,7 @@ module Producer
         end
 
         def task(name, &block)
-          @tasks << Task.new(name, &block)
+          @tasks << Task.evaluate(name, env, &block)
         end
       end
     end