Target .NET Standard 2.0

[bdovaz]

Please update the NetCore project to target .NET Standard 2.0.

I have tried to do it myself to make a PR but I have not been able to. Steps I have followed:

• Update the Rssdp.NetCore.csproj to the new csproj format.
• Make target to .NET Standard 2.0.
• Change #if NETSTANDARD1_3 to #if NETSTANDARD2_0
• Add the Nuget package Microsoft.CodeAnalysis.FxCopAnalyzers as the one used seems not to be compatible.

But since I don't know how it works FxCop has started running many rules that are not met in the project and makes not even compile if they are not fixed.