Added download.sh to download external dependencies

(cherry picked from commit 460d79b)

external/download.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+
+set -e
+
+cd $(dirname "$0")/..
+cat .gitmodules | \
+while true; do
+    read module || break
+    read line; set -- $line
+    path=$3
+    read line; set -- $line
+    url=$3
+    read line; set -- $line
+    branch=$3
+    git clone $url $path -b $branch --recursive
+done