Added support of Parallels provider

Related to GH-39

Since Packer is supporting Parallels now, I've made this template compatible with `parallels-iso` builder.
I've also tested template and builded a box-file for `parallels` provider: http://download.parallels.com/desktop/vagrant/boot2docker.box

@mitchellh, you can add this link to your box on [Vagrantcloud](https://vagrantcloud.com/mitchellh/boot2docker), I think it will be very useful for our users.