Convert to .NET Standard.

[paveloctober]

Library should be converted to .NET Standard in order to be supported by:

• .NET Core;
• .NET Framework;
• Mono;
• Xamarin.iOS;
• Xamarin.Mac;
• Xamarin.Android;
• Universal Windows Platform.

Please read article for more details:

• https://devblogs.microsoft.com/dotnet/introducing-net-standard/

[dima-hx]

Done in fork https://github.com/dima-horror/webtopay-lib-dotnet

Published as nuget package https://www.nuget.org/packages/paysera-bank-client/