Create a plugin for up'ing a lo device

- Believe we need sudo to create netns
- Use syscall instead of relying on ip netns
- Add sudo to .travis.yml
- Needs more -E
- Revert Godeps GoVersion to 1.4.2
- in travis, test command is run with all necessary env vars
- Loopback plugin only works on 'lo' interface
- Update README, add loopback plugin config
- note script dependency on jq

Signed-off-by: Gabe Rosenhouse <grosenhouse@pivotal.io>

plugins/main/loopback/loopback.go

@@ -0,0 +1,40 @@
+package main
+
+import (
+	"os"
+
+	"github.com/appc/cni/pkg/ns"
+	"github.com/appc/cni/pkg/skel"
+	"github.com/vishvananda/netlink"
+)
+
+func cmdAdd(args *skel.CmdArgs) error {
+	args.IfName = "lo" // ignore config, this only works for loopback
+	err := ns.WithNetNSPath(args.Netns, false, func(hostNS *os.File) error {
+		link, err := netlink.LinkByName(args.IfName)
+		if err != nil {
+			return err // not tested
+		}
+
+		err = netlink.LinkSetUp(link)
+		if err != nil {
+			return err // not tested
+		}
+
+		return nil
+	})
+	if err != nil {
+		return err // not tested
+	}
+
+	return nil
+}
+
+func cmdDel(args *skel.CmdArgs) error {
+	// del does nothing, we're going to destroy the device anyway
+	return nil
+}
+
+func main() {
+	skel.PluginMain(cmdAdd, cmdDel)
+}