Modified to enabled original src paths to be maintained in destination.

I wanted the same directory structure to be recreated at destination as was found at the source which I don't think currently happens. I took a look at how svgmin does it and managed to create a solution. It does mean you have to set ```cwd``` in the config in order to separate it from the source files. 

```
files: [
    {
        cwd: 'src/img/',
        src: '**/*.svg',
        dest: 'build/img/'
    }
]
```

It basically uses ```this.files``` instead of ```this.data.files``` which already has a reference to the globbed files and gives you the ability to then keep the ```cwd``` part separate meaning you can then recreate the full directory path in the destination.

Also, I think this will solve the issue which is being talked about [here](https://github.com/dbushell/grunt-svg2png/issues/4)