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).

build

@@ -11,6 +11,12 @@ fi
 export GOBIN=${PWD}/bin
 export GOPATH=${PWD}/gopath:$(pwd)/Godeps/_workspace
 
+echo "Building API"
+go build ${REPO_PATH}/libcni
+
+echo "Building reference CLI"
+go install ${REPO_PATH}/cnitool
+
 echo "Building plugins"
 
 PLUGINS="plugins/meta/* plugins/main/* plugins/ipam/*"