Re-design the tag fetching mechanism

[LCaparelli]

Currently, the tags are fetched using github.com/heroku/docker-registry-client/registry, which does not export its client as an interface. This means we can't easily mock a client by simply implementing an interface, which would come in very handy for testing.

We could wrap their client into an interface of our own and use that for production code as well as in testing, thus allowing us to create a mock registry client from that interface as well.

That would give us the power to conduct deterministic tests by emulating the registry client. This granularity would allow us to test the errors and achieve 100% test coverage in the package.