tuf: add add-aci command

Basic workflow now looks like:

```
mkdir -p staged/targets/appc.io
cp ace-validator* staged/targets/appc.io
tuf add-aci appc.io/ace-validator-sidekick.aci
tuf snapshot; tuf timestamp; tuf commit
```

Now on the client side:

```
tuf-client list http://127.0.0.1:8000/repository
tuf-client init http://127.0.0.1:8000/repository  /tmp/keys
PATH                                 SIZE  CUSTOM
/appc.io/ace-validator-sidekick.aci  44MB  {"acKind":"ImageManifest","acVersion":"0.2.0","annotations":[{"name":"coreos.com/rocket/stage1/run","value":"/init"},{"name":"coreos.com/rocket/stage1/enter","value":"/enter"}],"labels":[{"name":"version","value":"0.0.1"},{"name":"arch","value":"amd64"},{"name":"os","value":"linux"}],"name":"coreos.com/rocket/stage1"}
```

cmd/tuf/add-aci.go

@@ -0,0 +1,49 @@
+package main
+
+import (
+	"encoding/json"
+	"os"
+
+	"github.com/appc/spec/aci"
+	"github.com/flynn/go-tuf"
+	"github.com/flynn/go-tuf/Godeps/_workspace/src/github.com/flynn/go-docopt"
+)
+
+func init() {
+	register("add-aci", cmdAddACI, `
+usage: tuf add-aci [--expires=<days>] [<path>...]
+
+Add target file(s).
+
+Options:
+  --expires=<days>   Set the targets manifest to expire <days> days from now.
+`)
+}
+
+func cmdAddACI(args *docopt.Args, repo *tuf.Repo) error {
+	var custom json.RawMessage
+	if c := args.String["--custom"]; c != "" {
+		custom = json.RawMessage(c)
+	}
+	paths := args.All["<path>"].([]string)
+	f, err := os.Open("staged/targets/" + paths[0])
+	if err != nil {
+		panic(err)
+	}
+	img, err := aci.ManifestFromImage(f)
+	if err != nil {
+		panic(err)
+	}
+	custom, err = img.MarshalJSON()
+	if err != nil {
+		panic(err)
+	}
+	if arg := args.String["--expires"]; arg != "" {
+		expires, err := parseExpires(arg)
+		if err != nil {
+			return err
+		}
+		return repo.AddTargetsWithExpires(paths, custom, expires)
+	}
+	return repo.AddTargets(paths, custom)
+}