Use Decode/Encode in etcd_tools
All types stored in etcd are now API objects.

This means I made an api.ContainerManifestList, and added a JSONBase to
endpoints (and changed Name -> JSONBase.ID).
Incorporate new types into versioned api system.

* Made externalize/internalize generic to prevent boilerplate.
* Add fuzz testing.
* All objects pass fuzz tests now.
* This turned up some things we'll need to fix eventually. Left TODOs.
Merge pull request #571 from lavalamp/master

Decode/Encode everywhere