Add support for return orders

[racke]

This is a very important feature for merchants.

Regards Racke

[racke]

Some ideas about the needed logic:

• only shipped orders
• shipping / received date less than 14 days in the past
• customer can backorder individual items (e.g. 1 out of 3 qty)
• customer is required to supply backorder reason
• backorders are saved in the database

[SysPete]

So we start with two result classes:

• Return
• ReturnProduct (or ReturnLine)

Most methods should be in Return class/resultset.