Add AppVeyor CI integration.

AppVeyor is a cloud-based CI platform for windows.
Use [skip ci] tag in any place of message to mark commit as trivial (i.e.
README.md changes).

appveyor.yml

@@ -0,0 +1,40 @@
+environment:
+  priv_key:
+    secure: <encryped-value>
+
+install:
+  - ps: $fileContent = "-----BEGIN RSA PRIVATE KEY-----`n"
+  - ps: $fileContent += $env:priv_key.Replace(' ', "`n")
+  - ps: $fileContent += "`n-----END RSA PRIVATE KEY-----`n"
+  - ps: Set-Content c:\users\appveyor\.ssh\id_rsa $fileContent
+  - git config --global url.git@github.com:.insteadOf https://github.com/
+  - git submodule update --init --recursive -- src/monad src/windows-build test/Pester
+  - ps: Invoke-WebRequest -Uri https://dotnetcli.blob.core.windows.net/dotnet/dev/Binaries/Latest/dotnet-win-x64.latest.zip -OutFile dotnet.zip
+  - 7z x dotnet.zip -odotnet > nul
+
+build_script:
+  - ps: Set-Alias -Name dotnet -Value "$pwd\dotnet\bin\dotnet.exe"
+  - ps: dotnet restore
+  - ps: .\build.ps1
+
+test_script:
+  - ps: |
+      $testResultsFile = ".\TestsResults.xml"
+      .\bin\powershell.exe -c "Invoke-Pester test/powershell -OutputFormat NUnitXml -OutputFile $testResultsFile"
+      (New-Object 'System.Net.WebClient').UploadFile("https://ci.appveyor.com/api/testresults/nunit/$($env:APPVEYOR_JOB_ID)", (Resolve-Path $testResultsFile))
+
+
+deploy_script: 
+  - ps: |
+      # Creating project artifact
+      $zipFilePath = Join-Path $pwd "$(Split-Path $pwd -Leaf).zip"
+      Add-Type -assemblyname System.IO.Compression.FileSystem
+      [System.IO.Compression.ZipFile]::CreateFromDirectory("$pwd\bin", $zipFilePath)
+      
+      @(
+          # You can add other artifacts here
+          $zipFilePath
+      ) | % { 
+          Write-Host "Pushing package $_ as Appveyor artifact"
+          Push-AppveyorArtifact $_
+        }