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.
Replace ":" with "_" in png path

Mac OSX 10.7 (Lion) can't have colons in a filename.
Add chaching location of PNG files
getState now includes track id for easier polling

track_id is now part of the JSON of getState. This commit also extracts
getState out to a seperate file, making maintainance eaiser.
Refactor isRunning

After more time with AppleScript I found a lot nicer method of determining
whether Spotify is running or not.

This shrinks `isRunning` down to one, easy-to-understand line, using `running`
attribute of the application Spotify. Apparently every application has a
`running` attribute, which I didn't know about before.
Merge pull request #11 from rmehner/get_state_improvements

getState now includes track id for easier polling
Merge pull request #12 from mrnugget/refactor_is_running

Refactor isRunning
Merged master