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/task/dsl.rb

@@ -3,6 +3,12 @@ module Producer
     class Task
       class DSL
         class << self
+          def evaluate(name, env, &block)
+            dsl = new(&block)
+            dsl.evaluate(env)
+            Task.new(name, dsl.actions)
+          end
+
           def define_action(keyword, klass)
             define_method(keyword) do |*args|
               @actions << klass.new(@env, *args)