Fixed bug credits feature

- fixed Undefined property: CitruscartViewOpc::$userinfo
site\com_citruscart\controllers\checkout.php
- fixed bug when delete credits in admin area
admin\com_citruscart\tables\credits.php
change '/' to used .DS. in admin defines.php
fixed bug shipping feature in opc

fixed bug when used shipping feature
invalid json response data in one page checkout
and not show data in order review section
Merge pull request #7 from kankai34/master

fixed bug 2 commit and change DS