Shelljoin on windows correctly

I noticed that vagrant-wrapper 
* works correctly if you run `vagrant up --provider virtualbox`
* fails if you run `vagrant up --provider=virtualbox` (notice the `=`)

The error I get with vagrant-wrapper's `vagrant.bat` in front of the PATH is:
```
C:\some\path>vagrant up --no-provision --provider=virtualbox
An invalid option was specified. The help for this command
is available below.

Usage: vagrant up [options] [name]

Options:

        --[no-]provision             Enable or disable provisioning
        --provision-with x,y,z       Enable only certain provisioners, by type.
        --[no-]destroy-on-error      Destroy machine if any fatal error happens (default to true)
        --[no-]parallel              Enable or disable parallelism if provider supports it
        --provider PROVIDER          Back the machine with a specific provider
    -h, --help                       Print this help
```

The same command works with plain `vagrant.exe` though. 

For more context see tknerr/sample-toplevel-cookbook#5 where the issue occured.

It seems to be because of using `Shellwords.join(args)` which is intended for [escaping according to UNIX bourne shell rules](http://ruby-doc.org/stdlib-2.0/libdoc/shellwords/rdoc/Shellwords.html) but obviosuly does not work properly on windows.

A first workaround is to bypass `Shellwords.join` completely on Windows. This makes my use case in tknerr/sample-toplevel-cookbook#5 working again, but some more testing would be helpful.

I added back a bit of the shell join functionality by using [this dos_escape](https://github.com/rubyworks/facets/issues/17). My use case still works with this, but again, some more test cases would be helpful.

Another approach would be to use larskanis/shellwords, which seems to provide the functionality for both unix and windows. However, I refrained from that as it could potentially affect the existing behaviour for Linux users. Right now it will only affect Windows users.

@btm wants to chime in?