Unify datetime column default values between MySQL and Postgres.

Datetime columns should have 'default' => null, in both Postgres and
MySQL.

Fixes #3837
Fix a race condition problem

Prevents Model::save() from generating a query with WHERE 1 = 1 on race condition.

Refs #3857
Fix CS
Merge pull request #3959 from chinpei215/master-issue3857-fix

Fix a race condition problem

Fixes #3857
Update CONTRIBUTING.md

Added some helpful links. Updated PHPUnit version requirement.
Merge pull request #3976 from davidyell/patch-2

Update CONTRIBUTING.md
Merge branch 'master' of github.com:cakephp/cakephp