mix.ps1 Revision

This revises `mix.ps1` to:

* Use `&` instead of `cmd /C` to call the batch file
* Modify and use `$args` directly instead of building a string which represents the final command
* Represent the path to `mix.bat` as a FileInfo object

This was done mainly to fix quoting/spacing issues which generated similar errors as with the `call %~dp0` saga (ex. 'C:\Program is not recognized`).