Use ISO standard for hour format.

Resolves https://github.com/cakephp/cakephp/issues/2090

After already complying to ISO standards regarding localization CakePHP 3.0 should comply with the international standard for hour format as outlined in ISO 8601: 

> ISO 8601 uses the 24-hour clock system

This is now the right time to adjust it.
Related and dependent issues around DateTime as object etc. have already been resolved.