variable 7.x-2.1

Release notes

Important notes:
- If using Internationalization(i18n), this version of Variable requires i18n > 7.x-1.6
- Fixed multiple issues blocking automatic updates of variable + i18n, http://drupal.org/node/1666670

Full list of changes since 7.x-2.0:
4da6661 Issue #1635984 by Cyberwolf: Fixed error: undefined function variable_callback() when calling variable_get_value() for a variable with value callback set.
5e3daa1 Issue #1643512: Fixed Fatal error: Call to a member function getDefaultWeight() on a non-object in variable_realm().module on line 548.
77004ef Issue #1649928: Fatal error: Call to undefined method VariableRealmDefaultController::setWeight().
2092dab Issue #1664154 by Jose Reyero: Fix variable hooks invoked during bootstrap (Variable realm breaks pathauto).
89890b6 Issue #1661022 by guillaumev: Fixed Can not add email variables to the language realm.
b86754d Issue #1663762 by guillaumev: Fixed Undefined index: #submit in variable_form_alter().