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).

test

@@ -11,7 +11,7 @@
 source ./build
 
 TESTABLE="plugins/ipam/dhcp"
-FORMATTABLE="$TESTABLE pkg/ip pkg/ns pkg/plugin pkg/skel plugins/ipam/host-local plugins/main/bridge plugins/meta/flannel"
+FORMATTABLE="$TESTABLE libcni pkg/ip pkg/ns pkg/invoke pkg/types pkg/ipam pkg/skel plugins/ipam/host-local plugins/main/bridge plugins/meta/flannel"
 
 # user has not provided PKG override
 if [ -z "$PKG" ]; then