Emit armor headers in sorted order

[andrewgdotcom]

closes #254

[lubux]

Thanks, looks good to me.

[twiss]

For completeness, in case this is something you care about, note that key serialization for entities with multiple user IDs is also not deterministic:

go-crypto/openpgp/keys.go

Line 780 in 5521d83

for _, ident := range e.Identities {

(and the same applies in SerializePrivate).

[andrewgdotcom]

key serialization for entities with multiple user IDs is also not deterministic:

We don't call that code in hockeypuck AFAICT. We have a standard sort order that gets applied to packet sequences before display in the index view, which we can also apply to armored output.