* improves error handling and reporting
* uses run_command to reduce code
* fails quicker on errors as opposed to return codes and tracebacks
* can now also specify the key as data versus needing to wget it from a file

Tests used heavily mocked version of the apt_key code, which meant that
it didn't properly test real life scenario.

Pretty straightforward.  Give it a URL with an exported GPG key for signing an
Apt repository.  It downloads it and will install it using apt-key.  It's even
smart enough to tell if it's already there (i.e. actually tells you if it
changed or not).