queueSpotify() Implementation

(Work in progress)

This needs to be tested by multiple parties before merging.. pulled in code from issue #24 and made some modifications.

In particular, I'm not sure where that "random" number comes from - on my system, it uses the same number (```10030020```) every time.

Todo:
- [ ] Output needs to be sanitized
- [ ] Linting problems

Sample output:
```
$ SONOS_HOST=172.17.105.103 node examples/playspotify.js
null [ { '$': { 'xmlns:u': 'urn:schemas-upnp-org:service:AVTransport:1' },
    FirstTrackNumberEnqueued: [ '45' ],
    NumTracksAdded: [ '1' ],
    NewQueueLength: [ '45' ] } ]
```