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/lease.go

@@ -28,7 +28,7 @@ import (
 	"github.com/vishvananda/netlink"
 
 	"github.com/appc/cni/pkg/ns"
-	"github.com/appc/cni/pkg/plugin"
+	"github.com/appc/cni/pkg/types"
 )
 
 // RFC 2131 suggests using exponential backoff, starting with 4sec
@@ -285,7 +285,7 @@ func (l *DHCPLease) Gateway() net.IP {
 	return parseRouter(l.opts)
 }
 
-func (l *DHCPLease) Routes() []plugin.Route {
+func (l *DHCPLease) Routes() []types.Route {
 	routes := parseRoutes(l.opts)
 	return append(routes, parseCIDRRoutes(l.opts)...)
 }