Carve out initial application structure

This changeset defines the application structure to be used for the http side
of the new registry. The main components are the App and Context structs. The
App context is instance global and manages global configuration and resources.
Context contains request-specific resources that may be created as a by-product
of an in-flight request.

To latently construct per-request handlers and leverage gorilla/mux, a dispatch
structure has been propped up next to the main handler flow. Without this, a
router and all handlers need to be constructed on every request. By
constructing handlers on each request, we ensure thread isolation and can
carefully control the security context of in-flight requests. There are unit
tests covering this functionality.

layerupload.go

@@ -0,0 +1,63 @@
+package registry
+
+import (
+	"net/http"
+
+	"github.com/gorilla/handlers"
+)
+
+// layerUploadDispatcher constructs and returns the layer upload handler for
+// the given request context.
+func layerUploadDispatcher(ctx *Context, r *http.Request) http.Handler {
+	layerUploadHandler := &layerUploadHandler{
+		Context: ctx,
+		TarSum:  ctx.vars["tarsum"],
+		UUID:    ctx.vars["uuid"],
+	}
+
+	layerUploadHandler.log = layerUploadHandler.log.WithField("tarsum", layerUploadHandler.TarSum)
+
+	if layerUploadHandler.UUID != "" {
+		layerUploadHandler.log = layerUploadHandler.log.WithField("uuid", layerUploadHandler.UUID)
+	}
+
+	return handlers.MethodHandler{
+		"POST":   http.HandlerFunc(layerUploadHandler.StartLayerUpload),
+		"GET":    http.HandlerFunc(layerUploadHandler.GetUploadStatus),
+		"PUT":    http.HandlerFunc(layerUploadHandler.PutLayerChunk),
+		"DELETE": http.HandlerFunc(layerUploadHandler.CancelLayerUpload),
+	}
+}
+
+// layerUploadHandler handles the http layer upload process.
+type layerUploadHandler struct {
+	*Context
+
+	// TarSum is the unique identifier of the layer being uploaded.
+	TarSum string
+
+	// UUID identifies the upload instance for the current request.
+	UUID string
+}
+
+// StartLayerUpload begins the layer upload process and allocates a server-
+// side upload session.
+func (luh *layerUploadHandler) StartLayerUpload(w http.ResponseWriter, r *http.Request) {
+
+}
+
+// GetUploadStatus returns the status of a given upload, identified by uuid.
+func (luh *layerUploadHandler) GetUploadStatus(w http.ResponseWriter, r *http.Request) {
+
+}
+
+// PutLayerChunk receives a layer chunk during the layer upload process,
+// possible completing the upload with a checksum and length.
+func (luh *layerUploadHandler) PutLayerChunk(w http.ResponseWriter, r *http.Request) {
+
+}
+
+// CancelLayerUpload cancels an in-progress upload of a layer.
+func (luh *layerUploadHandler) CancelLayerUpload(w http.ResponseWriter, r *http.Request) {
+
+}