Make rake task for code statistics handle new test locations properly

As of 2a68f68a:

- Unit tests are now in test/models, instead of test/units
- Functional tests are now in test/controllers, instead of
  test/functional
- Helper tests are now in test/helpers, instead of test/units/helpers
- Mailer tests are now in test/mailers, instead of test/functional

Update the rake task for code statistics (`rake stats`) so that it
recognizes files in the above locations as tests, and accurately
calculates statistics such as "Test LOC" and "Code to Test Ratio."
Safely handle Rails apps that still have tests in the old locations.

RG2 packager expects each spec.files path to be a file and bombs when it tries to tarball a dir.

May revert if rubygems/rubygems#413 is accepted.

Otherwise it was matching "end" lines from blocks within the method body,
making tests such as the migration ones fail.

Add `variables:` to database.yml for mysql, mysql2, and postgresql adapters

Kindle ebook generation with working section navigation

in the new 'variables:' hash in each database config section in database.yml.
The key-value pairs of this hash will be sent in a 'SET key = value, ...'
query on new database connections.

The configure_connection methods from mysql and mysql2 into are
consolidated into the abstract_mysql base class.

This reverts commit 83cb6fbd.

Reason: very trivial

[ci skip]

* Fix example queries
* Remove doc entries of where.like/not_like.
* Remove :chain from where.not related docs. To me that's an implementation
  detail and we don't expect people to use where(:chain).not.

Fix #assert_instance_method to also assert indented methods

use _action callbacks in actionmailer

Use attr_accessor for Rails.{application,cache,logger}

Update AR Changelog with correct example using includes [ci skip]

These queries don't seem to work without the includes clause. [ci skip]

Update guides to use _action callbacks [ci skip]

use :only instead of :except callback option in the controller template

Mailers are always async now, and we already discuss
queueing lower down in the guide.

There's no reason for guides to reference old behaviors. They should
be current as of the versions of Rails that they ship with, and including
older information just clutters thing.

I discussed this change with @fxn and he agrees.

Fix a typo in rdoc (*expect* for *except*)

As [Security Guide](http://edgeguides.rubyonrails.org/security.html#whitelists-versus-blacklists)
says, it's better to use `before_filter only: []` instead of `except: []`
so we don't forget to turn the filter off for newly added actions.

Actionize2

Actionize: Use `_action` callbacks in documentation and code

Rename ActiveSupport::BasicObject to ActiveSupport::ProxyObject

AS::BasicObject is used for proxy classes. Let's give it a less concerning
name. Also, it avoids the confusion with Ruby's Basic Object.