Implement getArtwork to get the path to the artwork as  PNG

This implements a getArtwork method that gets the current artwork as
PNG (from the actual TIFF Data that spotify returns). The conversion to
PNG happens because it's more compatible with current browsers and everything.

Example:

```javascript
var spotify = require('spotify-node-applescript');

spotify.getArtwork(function(err, artworkPath) {
    console.log('Current artwork is at %s', artworkPath);
});
```

Please note: The used applescript takes some time (0.5s to 1.5s on different
machines), so don't run the script too often (_not_ every second) or you
will run into strange errors. Especially don't run them in parallel.

Also the apple script is seperated into an external file, which makes it easier
to maintain IMHO.