Labels and extension metadata (fixed)

Labels and extension metadata (fixed)

The spec now has opaque 'annotations', although:

* It's a map[string]string and I would have prefered an
  opaque-in-the-runtime-spec type for an opaque-to-the-runtime
  field [1].
* It doesn't address arbitrary bundle content [2], but I have a
  separate tag for that [3].

[1]: https://github.com/opencontainers/specs/pull/331#issuecomment-192477931
[2]: Message-ID: <20151231045437.GA6362@odin.tremily.us>
     Subject: Re: Labels and extension meta data in containers #108
     Date: Wed, 30 Dec 2015 20:54:37 -0800
[3]: Message-ID: <20150826195447.GX21585@odin.tremily.us>
     Subject: Dropping the rootfs requirement and restoring arbitrary bundle content
     Date: Wed, 26 Aug 2015 12:54:47 -0700