Added configuration options to enable/disable SMS and Twitter integration.

This disables the IM, SMS and Twitter settings pages and queue handlers
depending on the config options.

config.php.sample

@@ -164,6 +164,12 @@ $config['sphinx']['port'] = 3312;
 // $config['memcached']['server'] = 'localhost';
 // $config['memcached']['port'] = 11211;
 
+// Disable SMS
+// $config['sms']['enabled'] = false;
+
+// Disable Twitter integration
+// $config['twitter']['enabled'] = false;
+
 // Twitter integration source attribute. Note: default is Laconica
 // $config['integration']['source'] = 'Laconica';