v3.28.0

• Added selection code for SalesOrders. #486 by @RemyBrok
• Add sync API endpoints #485 by @timothydc
• Add missing Item Average Cost attribute #477 by @lennartvdd
• Fix resources that require guid/primary key in an explicit GET parameter #478 by @lennartvdd
• Add original headers when using next page in Findable #457 by @tomdrissen
• Add docblocks about exceptions to Storable.php #456 by @MaartenWaegeman