Write one line with a summary of the commit

Create a default shipper for each supplier account

Files changed in commit:
PO_Header.php
Suppliers.php
sql/mysql/upgrade4.15-4.15.1.sql
On branch master

Create the new field defaultshipper in the suppliers table
Amend Suppliers.php to update the new field
When entering a purchase order default to the shipper defined
in this new field