Rename top level registry interface to namespace

Registry is intended to be used as a repository service than an abstract collection of repositories. Namespace better describes a collection of repositories retrievable by name.
The registry service serves any repository in the global scope.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

registry.go

@@ -10,8 +10,31 @@ import (
 	"golang.org/x/net/context"
 )
 
-// Registry represents a collection of repositories, addressable by name.
-type Registry interface {
+// Scope defines the set of items that match a namespace.
+type Scope interface {
+	// Contains returns true if the name belongs to the namespace.
+	Contains(name string) bool
+}
+
+type fullScope struct{}
+
+func (f fullScope) Contains(string) bool {
+	return true
+}
+
+// GlobalScope represents the full namespace scope which contains
+// all other scopes.
+var GlobalScope = Scope(fullScope{})
+
+// Namespace represents a collection of repositories, addressable by name.
+// Generally, a namespace is backed by a set of one or more services,
+// providing facilities such as registry access, trust, and indexing.
+type Namespace interface {
+	// Scope describes the names that can be used with this Namespace. The
+	// global namespace will have a scope that matches all names. The scope
+	// effectively provides an identity for the namespace.
+	Scope() Scope
+
 	// Repository should return a reference to the named repository. The
 	// registry may or may not have the repository but should always return a
 	// reference.