This reverts commit a40e3c1a.

Try to guess application's directory while trying to run the server, but only if config.ru is not present in current dir

After that commit, developers can set ENGINE_PATH in ENGINE/scripts/rails
file and load application's ./script/rails (most of the time it will be
dummy application used for testing). When running ./script/rails g it will
use application to boot up, but then it will use Engine's root and
configuration for generators.

With config.generators becomes a way to configure generators
for current instance only. For example:

module Blog
  class Engine < Rails::Engine
    config.generators do |g|
      g.orm :active_record
    end

    config.app_generators do |g|
      g.test_framework :rspec
    end
  end
end

such definition sets :active_record as orm for engine and :rspec
as test_framework for application. The values set with app_generators
can be overwritten in application using config.generators as you would
normally do:

module MyApp
  class Application < Rails::Application
    config.generators do |g|
      g.test_framework :test_unit
    end
  end
end

Sometimes it's easier to define routes inside Engine or
Application class definition (e.g. one file applications). The
problem with such case is that if there is a plugin that
has config/routes.rb file, it will trigger routes reload on application.
Since routes definition for application is not in config/routes.rb
file routes_reloader will fail to reload application's routes
properly. With this commit you can pass routes definition as a block
to routes method, which will allow to properly reload it:

class MyApp::Application < Rails::Application
  routes do
    resources :users
  end
end

Without that commit script_name always become '/', which
results in paths like //posts/1 instead of /posts/1

In order to run Engine as standalone application, you will need
Rails::Application instance in the same namespace that engine
one. It's very important to leave _railtie bound to whatever
used "namespace" method first.

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