extensible metadata patch

[Colin Walters <walters verbum org>]

I tossed up a patch for commits:
https://bugzilla.gnome.org/show_bug.cgi?id=707379

This intersects with what both of you are working on.  How about instead
of a separate .sizes file, we put "ostree.sizes" as a key in the
metadata?

For GPG signatures, that has to be "detached" metadata.  Something like
ostree.gpgsigs -> "aay", an array of binary blobs, based on what we get
from gpg.

Let me know what you think!