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/linux_only.txt

@@ -1,11 +0,0 @@
-plugins/ipam/dhcp
-plugins/main/bridge
-plugins/main/host-device
-plugins/main/ipvlan
-plugins/main/loopback
-plugins/main/macvlan
-plugins/main/ptp
-plugins/main/vlan
-plugins/meta/portmap
-plugins/meta/tuning
-plugins/meta/bandwidth