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

plugins/windows_only.txt

@@ -0,0 +1,4 @@
+plugins/ipam/host-local
+plugins/main/windows/win-bridge
+plugins/main/windows/win-overlay
+plugins/meta/flannel