Improve poor security recommendation in docs

As reported in #9960, the current documentation recommends an insecure practice for
key generation from a password (a single round of SHA-256). The modified documentation
uses ActiveSupport::KeyGenerator to perform proper key stretching.
Merge pull request #9980 from stouset/patch-1

Improve poor security recommendation in docs

[ci skip]
fix travis links in readme

links were redirects
Module#delegate checks nilness rather that falsehood if :allow_nil is true, and avoids multiple evaluation of the target method

Notes:

1) I hope nilness is a word.

2) See rationale for avoiding multiple evaluation in a comment in the patch, credit goes to @jeremy for pointing out this gotcha in the existing implementation.

3) Embeds a little joke dedicated to @pixeltrix (it could be worse! :D).

References #10347.
adding test for the symbol refs
Don't reprocess the options hash on every request
pass over the code comments

* Highlights the requirement of an attributes method.
* Removes some details that depend on the implementation of the class including the module.
* Applies guidelines here and there.
fixes a test, and explains why AR::AttributeMethods checks defined?(@attributes) in some places
Making the retrieval of the cache store class a method, also wrote
comments for the expand_cache_key method.
Use new hash syntax in module delegation docs [ci skip]
Fix typo in serialized_attribute_test. [ci skip]
Merge pull request #10368 from demands/fix-typo

Fix typo in serialized_attribute_test. [ci skip]
Merge pull request #10361 from teeparham/patch-1

Fix travis links in readme [ci skip]
Merge pull request #10364 from wangjohn/refactoring_activesupport_cache

Making the retrieval of the cache store class a method
Delegate #unscope query method
StatementInvalid takes WrappedDatabaseException's place
Merge pull request #10372 from jeremy/statement-invalid-wraps-underlying-exception

StatementInvalid takes WrappedDatabaseException's place
Bump activerecord-deprecated_finders. Fixes #10304
Bump version to rc1
rails/master is now 4.1.0.beta