Plural forms of error messages

[miedzinski]

Some error messages should be using plural forms:

• CharField: min_length, max_length
• DecimalField: max_digits, max_decimal_places, max_whole_places
• FileField: max_length
• ListField: min_length, max_length
• ModelField: max_length

Right now these translations are broken in languages other than English, e.g. CharField's min_length given length 3 translates to Upewnij się, że pole ma conajmniej 3 znaków in Polish instead of Upewnij się, że pole ma conajmniej 3 znaki.

This can be solved using ngettext.