Update Header.java

changes for edi 850 inbound usps
Update PurchaseOrderItemGroup.java
Update ShipmentInformationGroup.java
Create ReferenceMessageGroup.java
Create MessageText.java
Update TermsOfSale.java
Update PurchaseOrderItemGroup.java
Merge pull request #9 from nreidelb/patch-7

changes for edi 850 inbound usps