Introduce `registry.Manager` abstraction + impl for GitHub

The vast majority of ksonnet apps will know about at least one registry,
the official ksonnet `incubator` registry. In general, managing
registries can involve fairly complex logic (e.g., resolving a reference
to a registry to a remote registry specification; finding all
libraries exposed by a registry; and so on).

This commit will introduce the `registry.Manager` abstraction, as well
as an implementation for registries hosted as GitHub repositories.