Provide i18n Support for text variables

Hi! Drupal7 does not support multilingual variables in the settings.php anymore. As far is I could find out this was the way to translate the Lightbox2 Textsettings in D6.

In D7 hook_variable_info() needs to be implemented in modules to provide translateable variables.

I attached a patch to add this hook. Maybe it helps somebody.

As this is my first patch on d.o feedback is highly appreciated (even if the patch itself is technically not very sophisticated :))