FIX: allow separate strings for periods dative

See issue #3456: https://github.com/silverstripe/silverstripe-framework/issues/3456

I assume that DateTest->testAgoInPast() will fail now, since the new strings are missing in the lang/en.yml.
These strings are:
Date:
  SECAGO
  SECSAGO
  MINAGO
  MINSAGO
  HOURAGO
  HOURSAGO
  DAYAGO
  DAYSAGO
  MONTHAGO
  MONTHSAGO
  YEARAGO
  YEARSAGO