Factor an API out into a module

This takes some of the machinery from CNI and from the rkt networking
code, and turns it into a library that can be linked into go apps.

Included is an example command-line application that uses the library,
called `cnitool`.

Other headline changes:

 * Plugin exec'ing is factored out

The motivation here is to factor out the protocol for invoking
plugins. To that end, a generalisation of the code from api.go and
pkg/plugin/ipam.go goes into pkg/invoke/exec.go.

 * Move argument-handling and conf-loading into public API

The fact that the arguments get turned into an environment for the
plugin is incidental to the API; so, provide a way of supplying them
as a struct or saying "just use the same arguments as I got" (the
latter is for IPAM plugins).

ip/cidr.go

@@ -15,23 +15,10 @@
 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)
@@ -62,25 +49,3 @@ func Network(ipn *net.IPNet) *net.IPNet {
 		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
-}