Vagrantcloud selfhosted

This PR extends the existing Vagrant Cloud post-processor with the ability to manage self-hosted boxes.
It does so by introducing a new optional setting `box_download_url'. Example usage:


    "post-processors": [
       [
         {
             "type": "vagrant",
             "include": ["boot2docker-vagrant.iso"],
             "vagrantfile_template": "vagrantfile.tpl",
             "output": "boot2docker_{{.Provider}}_{{timestamp}}.box"
        },
        {
             "type": "vagrant-cloud",
             "box_tag": "username/example",
             "access_token": "SOME-SECRET-TOKEN",
             "box_download_url": "http://downloads.example.tld/basebox/boot2docker_{{.Provider}}_{{timestamp}}.box",
             "version": "0.1.4"
        }
       ]
    ]