Add veewee parallels export command.

To test:

Build a Parallels box as usual.

````
bundle exec veewee parallels export testbox
````
(leave the `veewee` directory if using RVM so that the system Vagrant is used)
````
vagrant plugin install vagrant-parallels
vagrant box add testbox /path/to/veewee/testbox.box
````

Should work. `~/.vagrant.d/boxes/testbox/parallels` should contain `box.pvm`, `metadata.json`, and a machine `Vagrantfile.`