Un-force relative in format_date and more on i18n

Previously discussed in #961

I've removed the lines in question and tested format_date in en_US and ru locales. It works as intended, however there's one more thing, not strictly related to the #961.

Since there are languages with more interesting pluralization rules than English [1], I also used string formatting markers in singular form of some messages. This way it's less confusing to translators when the first plural form (when inflection is the same for 1 *and* 21, for instance) requires a formatting marker, but there's none in the original string. Although, in fact, it will work without this change, and it is only "cosmetic", I've never seen a hardcoded ``1`` in singular form.

[1]: http://unicode.org/repos/cldr-tmp/trunk/diff/supplemental/language_plural_rules.html