Adding "maintainer" to metadata.yaml

[Clint Byrum]

I've been doing some charm reviews, and I am starting to think we cannot
wait any longer to assign strong ownership of charms to non ~charmers
members.

While we definitely want to encourage everybody who is interested in
using juju to consider being a member of ~charmers on Launchpad, that
requires that you demonstrate the necessary trust and restraint to
"have root on everybody's boxes", and not everyone will want to have
that responsibility.

So we need to act quickly, and add a strong, explicit relationship between
users who are the "maintainer" of a charm. ~charmers will still of course
be empowered to fix the charms in any way necessary, but sponsoring the
maintainer's changes in should be the priority for ~charmers members.
Also, maintainers should feel a sense of ownership of the charm.

I'd like to start recording maintainer: in metadata.yaml now, even though
juju and the charm browser will effectively ignore this. I think the
format should be either a name/email, or a URL pointing to the user
or team's Launchpad page. Perhaps that can be extended to all OpenID
providers in the future.

Thoughts?