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

.travis.yml

@@ -20,6 +20,7 @@ matrix:
   fast_finish: true
 
 install:
+  - sudo apt-get install gcc-multilib gcc-mingw-w64 -y
   - go get github.com/onsi/ginkgo/ginkgo
   - go get github.com/containernetworking/cni/cnitool