Registry V2 HTTP route and error code definitions

This package, ported from next-generation docker regsitry, includes route and
error definitions. These facilitate compliant V2 client implementation. The
portions of the HTTP API that are included in this package are considered to be
locked down and should only be changed through a careful change proposal.
Descriptor definitions package layout may change without affecting API behavior
until the exported Go API is ready to be locked down.

When the new registry stabilizes and becomes the master branch, this package
can be vendored from the registry.

Signed-off-by: Stephen J Day <stephen.day@docker.com>