Provide better information about variables with Variable module

Variable is an API module that provides better information about variables, and some other utilities, see http://drupal.org/project/variable

This patch adds some more information for variables in Strongarm's Admin page, only when that variable module is enabled, thus it doesn't add any new dependency.

If the variable module is not enabled or we don't have any information for that variable, it defaults to the regular Strongarm variable display.

See the screenshot