This commits introduces:
* The apt_key type;
* The apt_key provider;
* Unit tests for the type;
* Beaker/acceptance tests for the type/provider.
The idea behind apt_key is that apt::key will simply become a wrapper
that uses apt_key. Being a native type/provider apt_key is a lot less
error prone than the current exec behaviour of apt::key and adds a few
nice bonuses like inventory capabilities for mcollective users.
