Added new solution and project files for .NET standard

[andywcoder]

The existing solution and project files have been left untouched. The new SDK style project files only target .NET standard 2.0. They could also target .NET 2.0, but I didn't want to mess around with that version.

I added .NET standard support only for the projects that supported that without any code changes.

The binaries are still stored in the release folder (in a sub-folder), so it would be quite easy to create nuget packages that target .NET 2.0 and .NET standard 2.0 at the same time.

[hansmbakker]

I suggest to put the content of AssemblyInfo.cs in the *.csproj files and remove the AssemblyInfo.cs files and the compile remove statements.

For the rest, good that you're doing this!

@andrewkirillov please note that somebody else already put a netstandard copy of your library on NuGet which doesn't work properly, I would like it more to have an official one :)

[andywcoder]

I'm glad you like it. I left the AssemblyInfo.cs files in there on purpose, so that it's still possible to build the original solution and project files (since they need the AssemblyInfo.cs files).

[hansmbakker]

In that case you can also use the <GenerateAssemblyInfo>false</GenerateAssemblyInfo> element to fix it.

[hansmbakker]

@Andy9FromSpace I just saw that @andrewkirillov wrote on the forum:

As the project is no longer maintained, the registration of new forum users was disabled

source: http://www.aforgenet.com/forum/viewtopic.php?f=2&t=4476

I'm afraid that it means that the chances that your PR will be accepted are small :(

@andrewkirillov could you please review this PR and update the nuget packages?