Added reboot provisioner

There's no easy and reliable way to run multiple provisioner blocks and reboot between them, as the speed of the guest and host come into play. It would be nice to have a way to reliably force a reboot that then blocks the vagrant process from continuing until the guest has finished rebooting.

This adds a "reboot" provisioner to vagrant-windows. Here's an example [Vagrantfile](https://gist.github.com/sneal/7155901) that makes use of a "reboot" provisioner.

Questions
1. Is this just insane or insanely useful?
2. Is there a better way to do this?
3. If it is useful, does it belong in vagrant-windows?

* this isn't a complete PR, its obviously missing tests