Remove error handling for evaluation errors
Remove Env#current_recipe attribute
Improve Recipe::DSL#initialize spec

  Fix current examples by testing assignations, and add missing example
about @tasks assignation.
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.