sub_customer sql cleanup based on
https://github.com/apmuthu/simpleinvoices/commit/ebc32faf35b9097aea573962be7ab1ccd878831c
Extra field is: parent_customer_id
typo in comment
en_GB lang.php missing strings added
nl_NL all missing strings added, other lang.php missing title strings added
Updated translation stats in lang/lang_check.txt
php -q lang_check.php > lang_check.txt
Merge pull request #68 from apmuthu/master

Lang Translations updated. sub_customer extension sql update.