added appveyor config for ci and tests

MQTTnet.sln

@@ -19,6 +19,7 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "MQTTnet.NetStandard", "Fram
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Build", "Build", "{67C28AC1-BC3A-420A-BE9C-FA2401431CF9}"
 	ProjectSection(SolutionItems) = preProject
+		appveyor.yml = appveyor.yml
 		Build\build.ps1 = Build\build.ps1
 		Build\MQTTnet.AspNetCore.nuspec = Build\MQTTnet.AspNetCore.nuspec
 		Build\MQTTnet.nuspec = Build\MQTTnet.nuspec

appveyor.yml

@@ -0,0 +1,9 @@
+version: 1.0.{build}
+image: Visual Studio 2017
+before_build:
+- cmd: dotnet restore
+build:
+  project: Frameworks/MQTTnet.NetStandard/MQTTnet.Netstandard.csproj
+  verbosity: minimal
+test_script:
+- cmd: dotnet test Tests/MQTTnet.Core.Tests/MQTTnet.Core.Tests.csproj