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
Display exceptions in text format for xhr request
Making proper_table_name take in options.

The options will specify the prefix and the suffix. Also, I'm moving the
method to be an instance method on the +Migration+ instance. This makes more
sense than being a class method on the +Migrator+ class because the only
place that uses it is on a +Migration+ instance (in a method_missing
hook). The logic for the Migrator shouldn't be doing any work to
calculate the table name, it should be the Migration itself.

Also made some small indentation fixes.
Deprecating passing strings as class name in fixtures.
Merge pull request #11960 from kirs/xhr-errors

Show exceptions in text format for xhr requests
typo [ci skip]
Merge pull request #11984 from wangjohn/proper_table_name

Making proper_table_name take in options.
Merge pull request #12000 from wangjohn/deprecating_class_name_string_in_fixtures

Deprecating passing strings as class name in fixtures.
Adding deprecation assertions for proper_table_name.

This prevents deprecation warnings from popping up.
Merge pull request #12008 from wangjohn/skipping_deprecated_migration_tests

Skip tests using deprecated +proper_table_name+ method.
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