Cope with other order total codes e.g. 'reward', 'credit'

[kevinb456]

During debugging a users problem and looking through their log file I saw the logged message;

2014-02-28 15:43:22 - Piwik OpenCart mod: unknown order total code 'reward'.

This is a message which my mod logs when it doesn't recognise a code from the order total array. Find out if this is a default OC code, or whether an extra one from some custom mod the customer had installed. Possibly code to cope with this code.

[kevinb456]

Also order total 'credit' from another user;

    Hello there [...] I just notice something is when an order is placed and there is customer credit deducted for order (default opencart customer credit function), there is error "Piwik OpenCart mod: unknown order total code 'credit' " in error log in backend. I don't think this is a bug because the credit amount matches discount from piwik report, so I don't put in issue part in your Github and just let you know. :)

Consider, etc.