Add LICENSE.txt; update copyright statements

LICENSE.txt

Makefile

downloads_for_sale.php

downloads_for_sale/tmpl/downloads.php

downloads_for_sale/tmpl/order_compact.php

downloads_for_sale/tmpl/order_long.php

downloads_for_sale/tmpl/product_compact.php

downloads_for_sale/tmpl/product_long.php

elements/vmdlorderstate.php

elements/vmorderstates.php

language/de-DE/de-DE.plg_vmcustom_downloads_for_sale.ini

language/de-DE/de-DE.plg_vmcustom_downloads_for_sale.sys.ini

language/en-GB/en-GB.plg_vmcustom_downloads_for_sale.ini

language/en-GB/en-GB.plg_vmcustom_downloads_for_sale.sys.ini