Refactor and simplify recipe DSL evaluation usages:

Remove most of recipe evaluation code in Recipe class, and rely on
Recipe::DSL to get evaluated recipes.

* Remove Recipe#evaluate call from CLI, rely on
  Recipe.evaluate_from_file to get the evaluated recipe;
* Implement Recipe.evaluate_from_file(filepath, env);
* Implement Recipe::DSL.evaluate(code, env);
* Remove code and filepath accessor on Recipe;
* Remove Recipe.from_file and Recipe#evaluate methods;
* Move task evaluations in Recipe::DSL#evaluate;
* Modify Recipe constructor so that it accepts tasks as argument.

lib/producer/core/recipe/dsl.rb

@@ -4,6 +4,11 @@ module Producer
       class DSL
         attr_reader :tasks
 
+        def self.evaluate(code, env)
+          dsl = new(code).evaluate(env)
+          Recipe.new(dsl.tasks)
+        end
+
         def initialize(code = nil, &block)
           @code   = code
           @block  = block
@@ -17,6 +22,7 @@ module Producer
           else
             instance_eval &@block
           end
+          @tasks.each { |e| e.evaluate env }
           self
         end