WIP: Describe the new client <-> registry API in Swagger

This work-in-progress branch has my encoding of the #9015 API in
Swagger (see docker/docker-registry#627).  It also pulls in a bit of
the manifest schema from #8093, since the API handles manifest
content.  I've been trying to stick to the spec (even where I don't
agree with it) for now, with the exception that I've used /v3 for
basePath to avoid [the][1] [conflict][2] for:

    /v2/tags/some/tags/list

which get's interpreted as `/v2/tags/<name=some/tags/list>` in the in
the #8320 API but `/v2/<name=tags/some>/tags/list` in this API.

While we're kicking this around, you can [preview it in the Swagger
UI][3].

[1]: https://github.com/docker/docker/issues/9015#issuecomment-63178287
[2]: https://github.com/docker/docker/issues/9015#issuecomment-63197025
[3]: http://petstore.swagger.wordnik.com/ui/?url=https://rawgit.com/wking/docker/client-registry-api-spec/registry.json