Bug Fixes, API Async Calls - v1.0.2

• Async HTTP calls for UPS and USPS APIs
• FedEx bug fixes

.NET Standard 2.0 Support

• Support for .NET Standard 2.0
• Async RateManager.GetRatesAsync methods added

Upgrade FedEx WDSL to v26, return errors from FedEx

• Upgrade FedEx WDSL to v26
• FedEx errors are returned in the Shipment.Errors
• Breaking change from DotNetShipping: Shipment.ServerErrors renamed to Shipment.Errors
• Breaking change from DotNetShipping: USPSError renamed to Error (but fields stay the same)