DateIntervalMapping i18n

* implement a "since xxx" construct common in Fr
* allow several words per language for "present"
* adding more languages in config file
* custom (textual) separators for DateIntervalMapping (language dependant): allows for "de ... à ..." used in Fr
* create unit tests
I think I tried with an empty string in the first place but had a harder time trying to match it (I'm still not that much familiar with Scala).
Anyway I do not have a computer at hand now but will try it when I'm back and keep you posted.