Support releasing Vagrant VirtualBox images

I originally wrote [a Packer plugin](https://github.com/lmars/packer-post-processor-vagrant-s3) to do this but I kept getting timeout errors when uploading to S3 via the `goamz` package (see [this issue](https://github.com/lmars/packer-post-processor-vagrant-s3/issues/2)).

Since the `goamz` package is seemingly unreliable, I've chosen to use `s3cmd` to upload the box to S3, and then added `flynn-release vagrant -b BUCKET` which will scan the bucket for any boxes not in the manifest (`vagrant/flynn-base.json`) and add them in, referring to their metadata to determine the provider, version and SHA256 checksum.

This will get called like:

```
script/release-vm-images \
  20141011.0 \
  https://s3.amazonaws.com/flynn/ubuntu/pool/main/f/flynn-host/flynn-host_20140817.0~19e570e1_amd64.deb
```

with the two arguments likely coming from the output of running `script/release-flynn`.
s/Url/URL/