implement translation behavior default locale

Unless explicitly overriden, the behavior now reads I18n::defaultLocale() to find the locale to return translations in.

Bundled with this change set is adding support to the translate behavior
for unhydrated result sets

Closes #4509

Ref #4660 
Note that [the app bootstrap file always sets intl.default_locale](https://github.com/cakephp/app/blob/master/config/bootstrap.php#L94) - you might be missing that line with an older project. To the translate behavior, this is the locale to put directly in your widgets table and  _not_ put in the i18n table. 

Are you specifically wanting your default locale content to be in the 18n table? You can force that (with e.g. `ini_set('intl.default_locale', 'xx');` in your app bootstrap file) - but it isn't how the translate behavior is designed to work.

I'm going to get rid of the hardcoded en_US default in i18n which is partially responsible for what you first observed.