Add Salt Provisioner  

This pull request adds a  provisioner that allows Vagrants to be provisioned with salt

it is a conversion of an existing plugin: https://github.com/saltstack/salty-vagrant

I am implementing this out of the following issues:
 - https://github.com/saltstack/salty-vagrant/issues/72
 - https://github.com/mitchellh/vagrant/issues/1416

Let me know if I can make any changes to make this provisioner better , or If I added it wrong. 

It would be great to see this provisioner come pre-packed with vagrant as salt is a very popular  server configuration management tool, and the salty-vagrant plugin is extremely popular 

Actual credits go mostly to 
      salty-vagrant: https://github.com/akoumjian
      salt-bootstrap: https://github.com/s0undt3ch