Added configuration option to disable post-by-email.

This hides the relevant settings from the email settings page and
prevents maildaemon.php from processing email if the option is
disabled.

config.php.sample

@@ -164,6 +164,9 @@ $config['sphinx']['port'] = 3312;
 // $config['memcached']['server'] = 'localhost';
 // $config['memcached']['port'] = 11211;
 
+// Disable post-by-email
+// $config['emailpost']['enabled'] = false;
+
 // Disable SMS
 // $config['sms']['enabled'] = false;