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