Implements garbage collection subcommand

- Includes a change in the command to run the registry. The registry
  server itself is now started up as a subcommand.
- Includes changes to the high level interfaces to support enumeration
  of various registry objects.

Signed-off-by: Andrew T Nguyen <andrew.nguyen@docker.com>

docs/gc.md

@@ -0,0 +1,28 @@
+<!--[metadata]>
++++
+title = "Garbage Collection"
+description = "High level discussion of garabage collection"
+keywords = ["registry, garbage, images, tags, repository, distribution"]
++++
+<![end-metadata]-->
+
+# What Garbage Collection Does
+
+Garbage collection is a process that delete blobs to which no manifests refer.
+It runs in two phases. First, in the 'mark' phase, the process scans all the 
+manifests in the registry. From these manifests, it constructs a set of content 
+address digests. This set is the 'mark set' and denotes the set of blobs to *not*
+delete. Secondly, in the 'sweep' phase, the process scans all the blobs and if 
+a blob's content address digest is not in the mark set, the process will delete 
+it.
+
+
+# How to Run
+
+You can run garbage collection by running
+
+	docker run --rm registry-image-name garbage-collect /etc/docker/registry/config.yml
+
+NOTE: You should ensure that the registry itself is in read-only mode or not running at
+all. If you were to upload an image while garbage collection is running, there is the
+risk that the image's layers will be mistakenly deleted, leading to a corrupted image.