Add strftime() support to CakeTime::timeAgoInWords()

When an absolute date is generated CakeTime will automatically use
strftime() if the format string contains %. This allows for simpler
localization.

Refs #4550
Update tests for strfime() in timeAgoInWords()

Add test asserts for testTimeAgoInWordsWithFormat with strftime format
Merge branch 'relative-sprintf' into 2.6

Refs #4550