Windows Support

Patch for https://github.com/containernetworking/plugins/pull/85

+ Windows cni plugins are added
   (*) win-bridge (hostgw)
   (*) win-overlay (vxlan)
+ Windows netconf unit test
+ Fix appveyor config to run the test
+ Build release support for windows plugins

Address comments

From:
    - https://github.com/containernetworking/plugins/pull/85
    - 0049c64e3f

.appveyor.yml

@@ -6,15 +6,16 @@ environment:
 install:
   - echo %PATH%
   - echo %GOPATH%
-  - set PATH=%GOPATH%\bin;c:\go\bin;%PATH%
   - go version
   - go env
+  - ps: $webClient = New-Object System.Net.WebClient; $InstallPath="c:" ; $webClient.DownloadFile("https://raw.githubusercontent.com/jhowardmsft/docker-tdmgcc/master/gcc.zip", "$InstallPath\gcc.zip"); Expand-Archive $InstallPath\gcc.zip -DestinationPath $InstallPath\gcc -Force; $webClient.DownloadFile("https://raw.githubusercontent.com/jhowardmsft/docker-tdmgcc/master/runtime.zip", "$InstallPath\runtime.zip"); Expand-Archive $InstallPath\runtime.zip -DestinationPath $InstallPath\gcc -Force; $webClient.DownloadFile("https://raw.githubusercontent.com/jhowardmsft/docker-tdmgcc/master/binutils.zip","$InstallPath\binutils.zip"); Expand-Archive $InstallPath\binutils.zip -DestinationPath $InstallPath\gcc -Force;
+  - set PATH=%GOPATH%\bin;c:\go\bin;c:\gcc\bin;%PATH%
 
 build: off
 
 test_script:
   - ps: |
-      go list ./... | Select-String -Pattern (Get-Content "./plugins/linux_only.txt") -NotMatch > "to_test.txt"
+      go list ./... | Select-String -Pattern (Get-Content "./plugins/windows_only.txt") > "to_test.txt"
       echo "Will test:"
       Get-Content "to_test.txt"
       foreach ($pkg in Get-Content "to_test.txt") {