Po file checker does not take plurals into account

[amercader]

So we have all these errors that have gone undetected and cause issues like #2454:

https://gist.github.com/amercader/2e960075d1313c4a6f07

Also we are not defining correctly some of the plural strings using ungettext:

    subject = ungettext(
        "1 new activity from {site_title}",
        "{n} new activities from {site_title}",
        len(activities)).format(
                site_title=pylons.config.get('ckan.site_title'),
                n=len(activities))

should be

    subject = ungettext(
        "{n} new activity from {site_title}",
        "{n} new activities from {site_title}",
        len(activities)).format(
                site_title=pylons.config.get('ckan.site_title'),
                n=len(activities))