IRosettaStats current interface doesn't suit database/pofile.py very well

The current IRosettaStats interface doesn't suit database/pofile.py very well. If you look at the POFile class, you'll see stuff like:

    def messageCount(self):
        """See IRosettaStats."""
        return self.potemplate.messageCount()

    def currentCount(self, language=None):
        """See IRosettaStats."""
        return self.currentcount

    def updatesCount(self, language=None):
        """See IRosettaStats."""
        return self.updatescount

    def rosettaCount(self, language=None):
        """See IRosettaStats."""
        return self.rosettacount

Apart from the inconsistency (and note also that messageCount is /not/ exported in the IRosettaStats interface, so I don't know what it's for) the unused language argument is pretty bogus.

SteveA suggests instead having a getStatsFor(lang) API that returns an IRosettaStats object. Carlos apparently concurs.