GR init hook, when the site is offline

When the site is offline for maintenance, it will be better, if init hook of GR gracefully returns without any function invocation. The _menu_site_is_offline() may be checked in the if condition for this.

Reason:
The call _menu_item_is_accessible(menu_get_active_item()) invokes the hook_menu. This causes many modules like jstools, light box to add their java script using drupal_add_js().

The Drupal's Default theme function for the site offline page, includes all these java script files, So we end up serving many unwanted js files.

Of course, this can be easily corrected by overriding the theme function with a custom one.

Also, since we get a 503 response when the site is offline, GR may not be needed.