Move ActionController::Railties::RoutesHelpers and ActionMailer::Railties::RoutesHelper to AbstractController::Railties::RoutesHelpers

There were actually 2 problems with this one:
* script_name was added to options as a string and then it was used
  in RouteSet#url_for with usage of <<, which was changing the original
  script_name
* the second issue was with _with_routes method. It was called in RoutesProxy
  to modify _routes in view_context, but url_helpers in views is just delegating
  it to controller, so another _with_routes call is needed there

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

For view_context we need to initialize RoutesProxy in context of controller, not view, quick fix, I need to dig into it later

rake db:copy_migrations task, which copies railties migrations into application's db/migrate directory

ActiveRecord::Migration#copy allows to copy migrations from one place
to another, changing migrations versions and adding scope to filename.

For example:
ActiveRecord::Migration.copy("db/migrate",
        :blog_engine => "vendor/gems/blog/db/migrate")

will copy all migrations from vendor/gems/blog/db/migrate to db/migrate
with such format:

Versions of copied migrations will be reversioned to be appended after
migrations that already exists in db/migrate

mounted_helpers are a bit similar to url_helpers. They're automatically
included in controllers for Rails.application and each of mounted
Engines. Mounted helper allows to call url_for and named helpers for
given application.

Given Blog::Engine mounted as blog_engine, there are 2 helpers defined:
app and blog_engine. You can call routes for app and engine using those
helpers:

app.root_url
app.url_for(:controller => "foo")
blog_engine.posts_path
blog_engine.url_for(@post)

* added more tests for prefix generation
* fixed bug with generating host for both prefix and url
* refactored url_for method
* organized tests for prefix generation

A few examples:
url_for Blog::Engine, :posts_path
url_for Blog::Engine, @post
url_for Blog::Engine, :action => "main", :controller => "index"

Refactored tests for prefix generation and added test for url generation in regular class with default_url_options[:script_name] set

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

This requires knowledge about original SCRIPT_NAME and
the parent router. It should be pass through the env
as ORIGIAL_SCRIPT_NAME and action_dispatch.parent_routes

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

This technique is here to allow using routes from Engine in Application
and vice versa. When using Engine routes inside Application it should
generate prefix based on mount point. When using Engine routes inside
Engine it should use env['SCRIPT_NAME']. In any other case it should
generate prefix as env should not be even available.

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