Migrate to .Net Standard 2.0 - Fix #8

[Kahbazi]

No description provided.

[Mds92]

Please describe what you did?

[Kahbazi]

With .Net Standard we are able to write library once and use it across all .Net Environment (.Net Framework 4.6, .NetCore, Xamarin, ...), This link has more info.

I look at your code and if I understand correctly, the two projects MD.PersianDateTime and MD.PersianDateTime.Core are no different in implementation, the source code are the same, the only difference is their target which one in dotnetcore and the other is dotnetframework.

I add a class library project for .Net Standard and add source code from MD.PersianDateTime.Core. So now you have only one project, one source, one nuget package which can be used in both DotNetCore and DotNetFramewrok.

I also took the liberty to change the nuget version to 4.0.0, because DotNetStandard needs Visual studio 2017 v 15.4 and I think it is a major change.

[Mds92]

Thanks for your notice, But I have to support old .net framework.
If I add another project, I have to support 3 package,

1. MD.PersianDateTime for .Net Framework 2+
2. MD.PersianDateTime.Core for .Net Core 2+
3. MD.PersianDateTime.Standard for newest .Net Framework And .Net Core
   I don't want to expire Project 1
   But I'm going to add Standard project base on requests like Net Standard #8

Thanks anyway