Get rid of static_paths method and instead configure paths for ActionDispatch::Static in initializers

Include application's helpers and router helpers by default, but include engine's ones for controllers inside isolated namespace

It's based specifying application's script_name with:
Rails.application.default_url_options = {:script_name => "/foo"}

default_url_options method is delegated to routes. If router
used to generate url differs from the router passed via env
it always overwrites :script_name with this value.

I added integration tests for generating urls in Engine and application
and tweaked Engines to fully cooparate with new router's behavior:
* Rails.application now sets ORIGINAL_SCRIPT_NAME
* Rails.application also sets its routes as env['action_dispatch.parent_routes']
* Engine implements responds_to? class method to respond to all the
  instance methods, like #routes

Pass routes via env['action_dispatch.routes'], it's needed by routes to determine if it should generate prefix for mounted apps

Move singleton pattern to Railtie and remove Engine::Configurable and Application::Configurable in favor of unified Railtie::Configurable

We don't need to add railties initlaizers in Application as there is already done in Engine and it's called with super

Removed ActionDispatch::Static, but left empty MiddlewareStack to unify app method between Engine and Application

Signed-off-by: NJosé Valim <jose.valim@gmail.com>

Ensure routes are loaded only after the initialization process finishes, ensuring all configuration options were applied.