Working on NLS support to Exiv2 library

Actually, i'm working with Andreas to improve Exiv2 library, especially to add the Native Language Support.

I have backported the Exif tags translations strings from the old libexif library into Exiv2. Also all translation files (.po) from libexif have been merged with Exiv2 to reduce the jobs of future translators.

The NLS support will be available to the next Exiv2 release planed at end of december.