Workaround for pnquant failing on windows by allowing to control parallel processing

Adds option to specify the parallel limit - pngquant sometimes fails when working in parallel on windows
Additionaly throws a more meaningfull error if pngquant or optipng fails

Solves #69

To work around the windows issue, or just to control the parallelism, use this:
```
        imagemin: {
            options: {
                parallel: 1,
                pngquant: true
            },
            ...
        }            
```