apt::key: Redo apt::key, make it use apt_key.

Introducing a totally rewritten and tested apt::key. This commit also
patches the spec's of apt::source because it was passing in data that
is no longer allowed by the new validation rules in apt::key.

It does its best to not touch any other specs and where we touch them
only minimally to ensure that we're not introducing breaking changes.
Update the README about apt_key and apt::key.
Merge pull request #230 from daenney/apt-key-defined

Rework apt::key to use apt_key.