Add Play Next and Play Previous functionality

- Use `cmd-shift-right` to play the next song
- Use `cmd-shift-left` to play the previous song
- Both commands can be accessed via *Packages -> Spotify* in the menu
- Fix an error when `spotify.getTrack` couldn't retrieve the current track
Merge pull request #1 from guidokessels/master

Add Play Next Song and Play Previous Song functionality