Fix localization of message post dates

This will fix:
- plural rules may differ for some languages: in English we have two
  plural forms, and singular is for items = 1, but other languages have
  different rules (for instance Russian has 3 plural forms, and in French
  singular is for items <= 1).
- Let's make clear that in "Posted on %s" the %s is a date
Merge pull request #118 from mlocati/fix-message-post-date-localization

Fix localization of message post dates