Setup GNUPG for test env and add some specs

ENV["GNUPGHOME"] in the list model is now only set for development and production environment. This allows us to specify another homedir in the specs.

For a basic setup for the test environment a gpg1 keyring and keypair is added and the keys are migrated to gpg 2.1 - so the setup will work both for gpg1 and gpg2.1. Keys, keyring etc. live in spec/gnupg. If any of the gpg features is needed in the specs, ENV["GNUPGHOME"] has to be set to spec/gnupg using the set_test_gnupg_home method that is added to the spec helper.

The setup is used in the specs to test the key method of the list model.