Implement isRunning to check whether Spotify is running or not

This commit adds a `isRunning` method, that checks via AppleScript if
the Spotify application is running or not.

`isRunning` takes a callback as argument. The callback gets passed two
arguments: `error` and `response`. `error` is a possible error bubbling up from
node-applescript.

`response` is a boolean, indicating whether Spotify is running (true) or not
(false).

The included test assumes that Spotify is running (as does the whole test
suite) and expects `isRunning` to return `true`. I did not bother to include the
negative version of the test, since that would mean changing a lot of the test
suite (including quitting and restarting spotify). But the method has been
tested manually and works as expected.
Escape quotes when getting track information

If you have a song title that includes quotes the old version produced
a parse error. For example for "spotify:track:4EZz8Byhbjk0tOKFJlCgPB" you'll run
into this, because it includes 7" in the album title.

With this commit all the quotes in the values getting properly escaped.

This also extracts the get_track script to a seperate file as it makes
maintaining easier.
Merge pull request #8 from rmehner/escape_quotes

Escape quotes when getting track information
Merge pull request #7 from mrnugget/is_running

Implement isRunning to check whether Spotify is running or not
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