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/cidr.go

@@ -0,0 +1,86 @@
+// 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 (
+	"encoding/json"
+	"math/big"
+	"net"
+)
+
+// ParseCIDR takes a string like "10.2.3.1/24" and
+// return IPNet with "10.2.3.1" and /24 mask
+func ParseCIDR(s string) (*net.IPNet, error) {
+	ip, ipn, err := net.ParseCIDR(s)
+	if err != nil {
+		return nil, err
+	}
+
+	ipn.IP = ip
+	return ipn, nil
+}
+
+// NextIP returns IP incremented by 1
+func NextIP(ip net.IP) net.IP {
+	i := ipToInt(ip)
+	return intToIP(i.Add(i, big.NewInt(1)))
+}
+
+// PrevIP returns IP decremented by 1
+func PrevIP(ip net.IP) net.IP {
+	i := ipToInt(ip)
+	return intToIP(i.Sub(i, big.NewInt(1)))
+}
+
+func ipToInt(ip net.IP) *big.Int {
+	if v := ip.To4(); v != nil {
+		return big.NewInt(0).SetBytes(v)
+	}
+	return big.NewInt(0).SetBytes(ip.To16())
+}
+
+func intToIP(i *big.Int) net.IP {
+	return net.IP(i.Bytes())
+}
+
+// Network masks off the host portion of the IP
+func Network(ipn *net.IPNet) *net.IPNet {
+	return &net.IPNet{
+		IP:   ipn.IP.Mask(ipn.Mask),
+		Mask: ipn.Mask,
+	}
+}
+
+// like net.IPNet but adds JSON marshalling and unmarshalling
+type IPNet net.IPNet
+
+func (n IPNet) MarshalJSON() ([]byte, error) {
+	return json.Marshal((*net.IPNet)(&n).String())
+}
+
+func (n *IPNet) UnmarshalJSON(data []byte) error {
+	var s string
+	if err := json.Unmarshal(data, &s); err != nil {
+		return err
+	}
+
+	tmp, err := ParseCIDR(s)
+	if err != nil {
+		return err
+	}
+
+	*n = IPNet(*tmp)
+	return nil
+}