Add support for containers/storage

Add containers/storage as a backend type called 'oci-storage'.

The image's blobs are stored either as oci-storage layers (if they look
like archived layers) or (alongside the manifest and signatures) as
named big data items associated with the image.

Inspection data and record-keeping (so that we can remember which blobs
were layers and which weren't) are encoded as a JSON object which is
stored as the oci-storage image's metadata.

The type of compression which we detected around a layer blob when it
was imported is encoded as a field in a JSON object which is stored as
the oci-storage layer's metadata, and is applied if we need to export
the layer later.

If we find ourselves importing an image with the same ID as an image
that we already have, we return an error.  If we find ourselves
importing an image which contains layers which we already have, we flag
an error if its contents differ from the one we already have, otherwise
we accept it.

If we find ourselves importing an image that wants to be tagged with a
name that is already in use, the name is assigned to the new image and
the old image remains otherwise unmodified.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

.travis.yml

@@ -13,4 +13,6 @@
   addons:
     apt:
       packages:
+        - btrfs-tools
+        - libdevmapper-dev
         - libgpgme11-dev

Makefile

@@ -5,16 +5,17 @@ SKOPEO_REPO = projectatomic/skopeo
 SKOPEO_BRANCH = master
 # Set SUDO=sudo to run container integration tests using sudo.
 SUDO =
+BUILDFLAGS = -tags "btrfs_noversion libdm_no_deferred_remove"
 
 all: deps .gitvalidation test validate
 
 deps:
-	go get -t ./...
-	go get -u github.com/golang/lint/golint
-	go get github.com/vbatts/git-validation
+	go get -t $(BUILDFLAGS) ./...
+	go get -u $(BUILDFLAGS) github.com/golang/lint/golint
+	go get $(BUILDFLAGS) github.com/vbatts/git-validation
 
 test:
-	@go test -cover ./...
+	@go test $(BUILDFLAGS) -cover ./...
 
 # This is not run as part of (make all), but Travis CI does run this.
 # Demonstarting a working version of skopeo (possibly with modified SKOPEO_REPO/SKOPEO_BRANCH, e.g.