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
No, this is just to cope with the change in config file, from single string per language to a set of strings. In English "now" is sometimes used instead of "present". In some other languages there may be a term meaning " in activity" for persons and something different for things or places.