Update push and pull to registry 2.1 specification

Minimal implementation for 2.1 specification.  Does not include fallback, using the v2 route package, or any sort of refactoring.

ping @stevvooe @jlhawn 
Maybe it would be better to move `newManifest` below `CmdManifest` so the diffs are more comparable. The diff tool seems to think they are the same function modified, which might be confusing.