Correct support for plural forms in Concrete5_Helper_Date->timeSince()

In English (and in many other languages) he have two forms for each word: singular (eg: day) and plural (eg: days).
Anyway there are some languages that have just one or more than two plural forms (for example Russian has 3 plural forms): see http://translate.sourceforge.net/wiki/l10n/pluralforms
The correct use of plural forms would be to use the recently added `t2()` function.