Compare images

[jml]

Adds a new compare-images command:

$ ./compare-images ../service-conf/k8s/dev/ ../service-conf/k8s/prod/
Image                          dev                   prod
-----------------------------  --------------------  --------------------
quay.io/weaveworks/grafana     master-0fc7cc2        master-08fd09d
quay.io/weaveworks/prometheus  master-0fc7cc2        master-4fb2aed
quay.io/weaveworks/ui-server   master-2899c36        master-45d67b3
tomwilkie/prometheus           frankenstein-8a5ec1b  frankenstein-ebe5808
weaveworks/scope               master-1a1021c        master-14d0e4e

Also:

• creates a new package called kubedifflib
• moves most of kubediff into there:
  • the diffing bits are in _diff
  • the common kubernetes stuff in _kube
• updates KubeObject to use attrs

---

This change is

[tomwilkie]

Couple of questions / nits, but LGTM otherwise.

[jml]

PTAL.