Merge pull request #7961 from exviva/seconds_until_end_of_day

Add #seconds_until_end_of_day to DateTime and Time
Added STI support to init and building associations

Allows you to do BaseClass.new(:type => "SubClass") as well as
parent.children.build(:type => "SubClass") or parent.build_child
to initialize an STI subclass. Ensures that the class name is a
valid class and that it is in the ancestors of the super class
that the association is expecting.
add list of extracted plugins to rails4 guide
augment the rails4 guide with links to PRs
Merge pull request #8356 from senny/add_links_to_rails4_guide

augmented the Rails 4 guides with links back to Github
Merge pull request #8363 from Takehiro-Adachi/add-string-options-examples-to-currentpage-method

Added examples to current_page? method [ci skip]
Tiny refactor
Getting Started: Rails 3.2 -> Rails 4.

This is based on Rails 4, not Rails 3.2.
User assert_kind_of, invert assert_equal expectations
Getting Started: mac -> Mac OS X

It's properly called "Mac OS X"
Getting Started: remove reference to attr_accessible
Getting Started: Improve instructions on opening a terminal
Migrations: move massive paragraph out of intro.

Most of the guides have a few simple sentences describing what they will show
you at the top. This one had a few big paragraphs. I've moved those paragraphs
down to an introductory one, and written a new smaller one for the introduction.
This makes this guide more consistent with the others.
ActiveRecord -> Active Record

The proper name, as per the documentation guides, is 'Active Record.' I've
updated several references to it as 'ActiveRecord'.
ActionPack -> Action Pack.

The proper name has a space.
ActionMailer -> Action Mailer.

The proper name for the library has a space.
ActiveSupport -> Active Support

The proper name has a space.
ActionController -> Action Controller

The proper name has a space.
ActionView -> Action View

The proper name has a space.
Migrations: Fix opening bullets.

The wording of these was a bit off, so I fixed them.