Add Nuget Dependency for .NET Framework, or support .NET Core.

[buckstephenh]

The reference to System.Runtime.Remoting.Messaging.CallContext is a .NET Framework library and not contained in .NET Core 2.0, but the only dependency listed on nuget.org is log4net (>= 2.0.3).

log4net:ERROR [ParallelForwardingAppender] ErrorCode: GenericFailure. Failed in DoAppend
System.TypeLoadException: Could not load type 'System.Runtime.Remoting.Messaging.CallContext' from assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.
at Log4Net.Async.AsyncForwardingAppenderBase.get_HttpContext()
at Log4Net.Async.ParallelForwardingAppender.Append(LoggingEvent loggingEvent)
at log4net.Appender.AppenderSkeleton.DoAppend(LoggingEvent loggingEvent)

[rcollette]

Some work on moving this to .NET standard/core was begun but hadn't been merged because on initial trial of it, it wouldn't run on a MAC.

I've just started a dotnetcore project. While I can't guarantee this will be done in the next few weeks, I will most likely get to looking at that PR in more detail since it will benefit my companies projects.

[StevenRasmussen]

Looking for .Net Core / .Net Standard support as well.

[wjdavis5]

@SimplerSoftware -
https://github.com/wjdavis5/Log4Net.Async
I've got a nuget package

https://www.nuget.org/packages/Log4Net.Async.Standard/