Multiple VS2012 Editions

Adding support for VS2012 Premium, Ultimate, and Test Professional. Including iso filenames as defaults - all that needs to be set is:

* ['visualstudio']['edition'] = 'ultimate' # or premium, professional, testprofessional
* ['visualstudio']['source'] = 'http://fileshare:8080/visualstudio' # or c:/vagrant/cache

If you have your ISOs with the same filename at the directory specified by visualstudio:source, then the recipe will pick up the appropriate file automatically once you specify the edition.

Example Vagrantfile JSON:

```javascript
{
    "visualstudio" => {
        "source" => "c:/vagrant/cache",
        "edition" => "ultimate"
    }
}
```