Move initialization code from main.go to the registry package

This makes it easier to embed a registry instance inside another
application.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

docs/doc.go

@@ -1,3 +1,2 @@
-// Package registry is a placeholder package for registry interface
-// definitions and utilities.
+// Package registry provides the main entrypoints for running a registry.
 package registry