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).

plugins/ipam/dhcp/options_test.go

@@ -18,20 +18,20 @@ import (
 	"net"
 	"testing"
 
-	"github.com/appc/cni/pkg/plugin"
+	"github.com/appc/cni/pkg/types"
 	"github.com/d2g/dhcp4"
 )
 
-func validateRoutes(t *testing.T, routes []plugin.Route) {
-	expected := []plugin.Route{
-		plugin.Route{
+func validateRoutes(t *testing.T, routes []types.Route) {
+	expected := []types.Route{
+		types.Route{
 			Dst: net.IPNet{
 				IP:   net.IPv4(10, 0, 0, 0),
 				Mask: net.CIDRMask(8, 32),
 			},
 			GW: net.IPv4(10, 1, 2, 3),
 		},
-		plugin.Route{
+		types.Route{
 			Dst: net.IPNet{
 				IP:   net.IPv4(192, 168, 1, 0),
 				Mask: net.CIDRMask(24, 32),