Template engine improvements

[rogelio-o]

• Add TemplateEngine for each Router, not for the full App. In case the Router doesn't have TemplateEngine, the parent Router TemplateEngine will be used.
• The Template will not be loaded before the renderer call is called, it will be renderer responsibility to load the template before render it. So, the TemplateEngine will not need the private TemplateLoader.
• Implementation of a DevTemplateLoader to load the templates from local files for development purpose.
• A template is not given to the renderer. The renderer receives a templateName and the template loader changes from templateName to a loaded template object.
• Transform the renderer from a function to a class.

[rogelio-o]

The test coverture has decreased. Add test before merge to:

• HttpResponse
• App
• Router