Project updated to target .NET Standard 2.0 and async methods interception

[SachiraChin]

Package for this solution in nuget only supports .NET Core 2.2 which I tried to use in one of my class libraries. But it didn't work because my library was targetted for .NET Standard 2.0. By adding support for .NET Standard 2.0, this library will be available for large number of other project types as well.

This pull request contains changes only for project files to target .NET Standard 2.0 and made change in InvocationContext.cs to support libraries in .NET Standard 2.0.

This also contains interception for async methods, I have used Castle.Core.AsyncInterceptor for that. I've done my best to preserve way you implemented it, please let me know if you need any changes to it.