Add a type and provider to manage apt keys.

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.
Travis: Remove 3.0, add newer 3.x releases.

3.0 is just broken in tests, something is wrong with the module loading
rendering tests completely useless.

Start testing on Puppet 3.3 and 3.4.
Merge pull request #212 from daenney/apt-key

apt_key type/provider