Update remote scraping to grab "last modified" date for foreign layers

[tianon]

Here's an example diff for hello-world:

diff --git a/repos/hello-world/tag-details.md b/repos/hello-world/tag-details.md
index 551670458..41e5cfcd1 100644
--- a/repos/hello-world/tag-details.md
+++ b/repos/hello-world/tag-details.md
@@ -231,9 +231,11 @@ RUN cmd /S /C #(nop)  CMD ["cmd" "/C" "type C:\\hello.txt"]

 -	Layers:
 	-	`sha256:bce2fbc256ea437a87dadac2f69aabd25bed4f56255549090056c1131fad0277`
+		Last Modified: Tue, 13 Dec 2016 10:47:17 GMT
 		Size: 252.7 MB (252691002 bytes)
 		MIME: application/vnd.docker.image.rootfs.foreign.diff.tar.gzip
 	-	`sha256:b0b5e40cb939a7befa4e01212d6f65f30022bbd04b5f15985b45ce9cfd3fcabc`
+		Last Modified: Mon, 09 Oct 2017 19:23:15 GMT
 		Size: 141.8 MB (141758132 bytes)
 		MIME: application/vnd.docker.image.rootfs.foreign.diff.tar.gzip
 	-	`sha256:87377cdc7dd13b955024f290de8aa6606f7ce958a72091495e3c258c800bdaf5`
@@ -479,9 +481,11 @@ RUN cmd /S /C #(nop)  CMD ["cmd" "/C" "type C:\\hello.txt"]

 -	Layers:
 	-	`sha256:bce2fbc256ea437a87dadac2f69aabd25bed4f56255549090056c1131fad0277`
+		Last Modified: Tue, 13 Dec 2016 10:47:17 GMT
 		Size: 252.7 MB (252691002 bytes)
 		MIME: application/vnd.docker.image.rootfs.foreign.diff.tar.gzip
 	-	`sha256:b0b5e40cb939a7befa4e01212d6f65f30022bbd04b5f15985b45ce9cfd3fcabc`
+		Last Modified: Mon, 09 Oct 2017 19:23:15 GMT
 		Size: 141.8 MB (141758132 bytes)
 		MIME: application/vnd.docker.image.rootfs.foreign.diff.tar.gzip
 	-	`sha256:87377cdc7dd13b955024f290de8aa6606f7ce958a72091495e3c258c800bdaf5`

[tianon]

Here's what an example manifest looks like for these foreign layers:

{
   "schemaVersion": 2,
   "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
   "config": {
      "mediaType": "application/vnd.docker.container.image.v1+json",
      "size": 1853,
      "digest": "sha256:64cd2c7dc23a8ac2258f081d6fb3858debdd178341ab8d51ef8acc622127b6fd"
   },
   "layers": [
      {
         "mediaType": "application/vnd.docker.image.rootfs.foreign.diff.tar.gzip",
         "size": 252691002,
         "digest": "sha256:bce2fbc256ea437a87dadac2f69aabd25bed4f56255549090056c1131fad0277",
         "urls": [
            "https://go.microsoft.com/fwlink/?linkid=837858"
         ]
      },
      {
         "mediaType": "application/vnd.docker.image.rootfs.foreign.diff.tar.gzip",
         "size": 141758132,
         "digest": "sha256:b0b5e40cb939a7befa4e01212d6f65f30022bbd04b5f15985b45ce9cfd3fcabc",
         "urls": [
            "https://go.microsoft.com/fwlink/?linkid=860052"
         ]
      },
      {
         "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
         "size": 1660,
         "digest": "sha256:87377cdc7dd13b955024f290de8aa6606f7ce958a72091495e3c258c800bdaf5"
      },
      {
         "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
         "size": 958,
         "digest": "sha256:7d840042f15f5e431f1d62b2337774f2a41dfe0303c87fce3692fecc998a4f87"
      }
   ]
}

(from winamd64/hello-world:nanoserver@sha256:6168e1e1f8cd1388c34aa53e1095cb6f1e51960f27e9b091e7160b70591fac21)

[tianon]

So neat:

$ curl -fsSL 'https://go.microsoft.com/fwlink/?linkid=837858' | sha256sum
bce2fbc256ea437a87dadac2f69aabd25bed4f56255549090056c1131fad0277  -

From the JSON above:

      {
         "mediaType": "application/vnd.docker.image.rootfs.foreign.diff.tar.gzip",
         "size": 252691002,
         "digest": "sha256:bce2fbc256ea437a87dadac2f69aabd25bed4f56255549090056c1131fad0277",
         "urls": [
            "https://go.microsoft.com/fwlink/?linkid=837858"
         ]
      },