Add error message for including nokogiri in Gemfile for rake doc:guides [ci skip]

- When `rake doc:guides` is run from applications, it complains for
  presence of redcarpet if it not present in Gemfile
- Similarly it should complain about nokogiri
Removing instances of string class_names in fixtures.

Also, constantizing the default_fixture_model_name when it gets loaded
in from the file. Later, when the class_name is passed to a new
FixtureSet, a deprecation warning will occur if the class_name is a
string.
Changing deprecation_horizon to be Rails 4.2

Also, +ActiveRecord::Migrator.proper_table_name+ should actually have a
deprecation horizon of Rails 4.2 (not 4.1).
Move view_paths from AP to AV
Require only path_set && lookup_context instead of whole base
Move rendering from AP to AV
Add loading 'action_view' as part of rails/all
Move layouts to AV
Hook up AV::Rendering on AV intialization
Revert "Require only path_set && lookup_context instead of whole base"

This reverts commit 77bb03df2ca3687b290e9fe77d21007642f0329c.
Fix AV tests, I18nProxy was moved to AV
Fix eager_autoload for layouts

Name was singular, should be plural
Integrate ActionMailer with ActionView
Do not include AV::Rendering

It's already included in Layouts, makes no sense to doubly include it
Fix AP test suite after moving stuff to AV
Rename abstract_controller/rendering. to errors.rb

Since all rendering stuff was extracted to AV, the only thing that left was single class with error so file name wasn't relevant anymore
Revert "Rename abstract_controller/rendering. to errors.rb"

This reverts commit 6fe91ec5008838338e54ab8570f7c95ee0cdd8e8.
Create AbstractController::Rendering interface

This interface should be use when implementing renderers.
Remove 'api plugin' from docs

AbstractController::Rendering is module which defines thie api
No need for .rb extension