Update QuickBooks_IPP_Service_CreditMemo class

IPP v3 now supports adding credit memos.  I've added the required methods to the credit memo service class to make this possible as well as the other standard methods found in other QuickBooks_IPP_Service child classes.
Merge pull request #19 from hackley2/patch-1

Update QuickBooks_IPP_Service_CreditMemo class.