Add plugin code

This adds basic plugins.
"main" types: veth, bridge, macvlan
"ipam" type: host-local

The code has been ported over from github.com/coreos/rkt project
and adapted to fit the CNI spec.

ip/ipmasq.go

@@ -0,0 +1,66 @@
+// Copyright 2015 CoreOS, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ip
+
+import (
+	"fmt"
+	"net"
+
+	"github.com/appc/cni/Godeps/_workspace/src/github.com/coreos/go-iptables/iptables"
+)
+
+// SetupIPMasq installs iptables rules to masquerade traffic
+// coming from ipn and going outside of it
+func SetupIPMasq(ipn *net.IPNet, chain string) error {
+	ipt, err := iptables.New()
+	if err != nil {
+		return fmt.Errorf("failed to locate iptabes: %v", err)
+	}
+
+	if err = ipt.NewChain("nat", chain); err != nil {
+		if err.(*iptables.Error).ExitStatus() != 1 {
+			// TODO(eyakubovich): assumes exit status 1 implies chain exists
+			return err
+		}
+	}
+
+	if err = ipt.AppendUnique("nat", chain, "-d", ipn.String(), "-j", "ACCEPT"); err != nil {
+		return err
+	}
+
+	if err = ipt.AppendUnique("nat", chain, "!", "-d", "224.0.0.0/4", "-j", "MASQUERADE"); err != nil {
+		return err
+	}
+
+	return ipt.AppendUnique("nat", "POSTROUTING", "-s", ipn.String(), "-j", chain)
+}
+
+// TeardownIPMasq undoes the effects of SetupIPMasq
+func TeardownIPMasq(ipn *net.IPNet, chain string) error {
+	ipt, err := iptables.New()
+	if err != nil {
+		return fmt.Errorf("failed to locate iptabes: %v", err)
+	}
+
+	if err = ipt.Delete("nat", "POSTROUTING", "-s", ipn.String(), "-j", chain); err != nil {
+		return err
+	}
+
+	if err = ipt.ClearChain("nat", chain); err != nil {
+		return err
+	}
+
+	return ipt.DeleteChain("nat", chain)
+}