allows to specify the output format of CakeTime::timeAgoInWords

There's already a *format* option, but this one was used with the *end* option,
when there is [a cutoff point to no longer will use words](http://book.cakephp.org/2.0/en/core-utility-libraries/time.html#CakeTime::timeAgoInWords).

Default value remain the same as before : "%s ago"