Add custom Gettext loader to fix plural rules

[mlocati]

When the Zend I18N component loads language files with wrong or missing plural rules, concrete5 aborts with this Zend\I18n\Exception\RuntimeException exception:

Plural rule of merging text domain is not compatible with the current one

End users may have a very hard time debugging (and understanding) what's going on.

So, what about fixing automatically the number of plural rules if possible (or showing a better error message if an automatic fix is not possible)?

[mlocati]

Here's an example of the new error displayed to the users:

The language file
/application/languages/it_IT/LC_MESSAGES/messages.mo
for Italian (Italy) has 1 plural form instead of 2.

[mlocati]

PS: this PR doesn't add much overhead, since the translations are cached once they are succesfully loaded.