Integrate strong_parameters in Rails 4

Stronger wording in Contributing file

Explicitly tell users to read the guide before submitting code. re: #7677. Specify the name of the mailing list for rubyonrails-talk to help differentiate it from rubyonrails-core mailing list. 

week_start option added to rails app config

show love to contributors

Date.beginning_of_week thread local and beginning_of_week application config option added (default is Monday)

Remove 'assigned but unused variable' warning

Add extra documentation for password_field

I think this should be explicit as the password fields behaviour is inconsistent with other fields in this regard.
It had me scratching my head until I dug through the source code.

fix rdoc task for Action Mailer documentation

Convert all the guides into GFM

Markdown has footnote, but Redcarpet doesn't support it.

* Use the current heading by default
* If duplicate, then use #[parent]-[current] as the id

In Textile `*` would convert to `<strong>`, but in Markdown we have to
use `**` instead.