Use symbols instead of strings

ActiveSupport::Concern is used all over Rails
This PR will only create 3 new objects as keys are never recreated and
are not subject to garbage collection.

The strings were being uniquely created and garbage collected. I don't
have any performance numbers but this should be better than all of the
GC.
make default value for `:message` on `AM::Errors` explicit.
Removing deprecated rake tasks.

The `rake test:recent` and `rake test:uncommitted` tasks were
deprecated and are now being removed.
Making the rake file for tests easier to read.

I'm defining a new class which modularizes how the `rake test` tasks are
defined and invoked.
Adding documentation to the Rails initialization guide. [ci skip]
fix activerecord/examples/performance.rb#L101 syntax error
Merge pull request #11126 from tumayun/fix_activerecord_examples_performance_rb_syntax_error

fix activerecord/examples/performance.rb#L101 syntax error
mention the removal of `--builder` in 4.0 release notes.
Add DelegationError class. Rasied by delegation to a nil object
linking small rails 4 features image to a bigger one [ci skip]

View a bigger version onclick
https://github.com/rails/rails/issues/11100
Merge pull request #10828 from southpolesteve/delegation_error_class

Create DelegationError class
Merge pull request #11128 from edg3r/patch-1

linking small rails 4 features image to a bigger one
Merge pull request #11124 from wangjohn/removing_deprecated_rake_tasks

Removing deprecated rake tasks for tests
Move delegation error constant to inside Module
Review railties changelog [ci skip]
Removing deprecation message and silencing method.

The deprecation rake task is no longer useful because the deprecated
methods have already been removed.

The method for silencing stderr has been removed since it was only
useful for `rake test:uncommitted` which was deprecated and removed.
Improved grammar and replaced 'dbs' slang with 'databases'
Use activerecord 4.0.0 for reporting bugs using the template[ci skip]
Fixing links to accept_language plugin/middleware
Merge pull request #11137 from WvanLelyveld/master

There was a parenthesis in the guides, which resulted in the one link not working, and a second link not even showing. [ci skip]