Merge pull request #33 from daniellittledev/nullable-types

Update to build os image to Windows 2019

Enable Nullable compiler option for safe null checks

Improved error handling when Handler returns null instead if Task

Update GetTaskResult to support types derived from Task<T>

Fixes Command and Event handlers so they don't throw if registered with RegisterHandlers

Switch from Tabs to Spaces

Remove PackageTargetFallback property and clean properties for Test projects

Fix use of PackageTargetFallback which is a breaking change in .NET Core 2.0

Target update project to take advantage of NETStandard.Library 2.0.0

Add support for LightInject container

Fix project references after moving the StructureMap projects

Move StructureMap Projects into src

Add support for StructureMap

Update package descriptions