socket.io-client-sharp 4.0.0

[StephenHodgson]

• Closes #358
• Changed all projects to target both .net standard 2.0 and .net 4.6.2
  • Except SocketIO.Serializer.MessagePack
• Changed SocketIO constructor to include ISerializer
  • ISerializer defaults to first available based on target framework
    • .net 4.6.2 SocketIO.Serializer.NewtonsoftJson
    • .net Standard 2.0+ SocketIO.Serializer.SystemTextJson
• Changed EventHandlers delegates to invoke object arrays instead of JsonElement arrays