Vagrantfile: Add support for VMWare Fusion provider

As a user who has blown $150 on VMWare Fusion and vagrant-vmware, I
would like to use my new shiny to hack on Docker. Docker already has a
multi-provider Vagrantfile, so adding another one presents little risk.

Known Issues:

- The docker install of a new kernel breaks the Vagrant shared folder.
    - This seems to be because the VMWare hgfs module doesn't build
      against a 3.8 kernel.
    - I don't believe that shared folder support is actually in use