Automatically generate V2 API specification

This changeset provides data structures and definitions describing the routes
available in the V2 registry API. These route descriptors are structured to
provide automated registration, for creating routers, in addition to complete
documentation duty. It's also a possibility that this could be used to
enumerate test coverage for server implementation.

Using this functionality, we've also developed a template to automatically
generate and API specification for submission into docker core.

digest/digest.go

@@ -7,6 +7,7 @@ import (
 	"hash"
 	"io"
 	"io/ioutil"
+	"regexp"
 	"strings"
 
 	"github.com/docker/docker-registry/common"
@@ -36,6 +37,9 @@ func NewDigest(alg string, h hash.Hash) Digest {
 	return Digest(fmt.Sprintf("%s:%x", alg, h.Sum(nil)))
 }
 
+// DigestRegexp matches valid digest types.
+var DigestRegexp = regexp.MustCompile(`[a-zA-Z0-9-_+.]+:[a-zA-Z0-9-_+.=]+`)
+
 var (
 	// ErrDigestInvalidFormat returned when digest format invalid.
 	ErrDigestInvalidFormat = fmt.Errorf("invalid checksum digest format")
@@ -125,6 +129,8 @@ func (d Digest) Validate() error {
 		return ErrDigestUnsupported
 	}
 
+	// TODO(stevvooe): Use DigestRegexp to validate digest here.
+
 	return nil
 }