Add fake gpg-agent for unit-testing with GnuPG 2.1

GnuPG now requires an agent, even though it doesn't need it for
anything and there's no way to stop it trying to connect to it.

See: http://stackoverflow.com/questions/27459869/how-to-stop-gpg-2-1-spawning-many-agents-for-unit-testing
Added `binary_of source impl_type