Migrate 2.x Translations validation

[dereuromark]

Migrate https://github.com/dereuromark/cakephp-setup/blob/cake2/Console/Command/TranslationsShell.php

Validating PO files.

//EDIT Could also be moved to https://github.com/dereuromark/cakephp-translate

[dereuromark]

First part in a175771