Add resumable downloads

Since VM images can be fairly large and connections rather flaky, it would be
nice to support resumable downloads whereby, if a download is interrupted for
some reason, on the next attempt, it picks up where it left off.

To implement this, the following changes were made:

* The temporary download filename is now constructed from a SHA1 of the
  `box_url` instead of a timestamp. This allows separate invocations of
  Vagrant to 'share' the download-path if the URLs exactly match.

* Add `--continue-at -` option to `curl` which tells it to automatically resume
  downloading where it left off

* Modify the `recover` method in `box_add` to not remove the temporary
  download path if the download was interrupted

Known Issue:

* The progress on a resumed download will look a bit wonky in the sense that,
  it starts at 0% each time, instead of where it left off. Since Vagrant is
  pulling this directly from `curl`, this is more of an upstream issue.

Fixes #57

CHANGELOG.md

@@ -5,6 +5,7 @@ IMPROVEMENTS:
   - commands/provision: Add `--no-parallel` option to disable provider
     parallelization if the provider supports it. [GH-2404]
   - providers/virtualbox: Enable symlinks for VirtualBox 4.1. [GH-2414]
+  - core: Support resumable downloads [GH-57]
 
 BUG FIXES:
 

lib/vagrant/action/builtin/box_add.rb

@@ -1,3 +1,4 @@
+require "digest/sha1"
 require "log4r"
 
 require "vagrant/util/downloader"
@@ -15,7 +16,8 @@ module Vagrant
         end
 
         def call(env)
-          @temp_path = env[:tmp_path].join("box" + Time.now.to_i.to_s)
+          @download_interrupted = false
+          @temp_path = env[:tmp_path].join("box" + Digest::SHA1.hexdigest(env[:box_url]))
           @logger.info("Downloading box to: #{@temp_path}")
 
           url = env[:box_url]
@@ -40,6 +42,7 @@ module Vagrant
           rescue Errors::DownloaderInterrupted
             # The downloader was interrupted, so just return, because that
             # means we were interrupted as well.
+            @download_interrupted = true
             env[:ui].info(I18n.t("vagrant.actions.box.download.interrupted"))
             return
           end
@@ -82,7 +85,7 @@ module Vagrant
         end
 
         def recover(env)
-          if @temp_path && File.exist?(@temp_path)
+          if @temp_path && File.exist?(@temp_path) && !@download_interrupted
             File.unlink(@temp_path)
           end
         end

lib/vagrant/util/downloader.rb

@@ -37,7 +37,8 @@ module Vagrant
           "--location",
           "--max-redirs", "10",
           "--user-agent", USER_AGENT,
-          "--output", @destination
+          "--output", @destination,
+          "--continue-at", "-"
         ]
 
         options << "--insecure" if @insecure