When plugin errors out, it prints out a JSON object to stdout
describing the failure. This object needs to be propagated out
through the plugins and to the container runtime. This change
also adds Print method to both the result and error structs
for easy serialization to stdout.
This adds basic plugins.
"main" types: veth, bridge, macvlan
"ipam" type: host-local
The code has been ported over from github.com/coreos/rkt project
and adapted to fit the CNI spec.
