I did the following using this guide: http://www.mabishu.com/blog/2010/11/15/playing-with-d-bus-interface-of-spotify-for-linux/
You will need to install mdbus2:
sudo apt-get install mdbus2
Now you can try out to play/pause:
dbus-send --print-reply --dest=org.mpris.MediaPlayer2.spotify /org/mpris/MediaPlayer2 org.mpris.MediaPlayer2.Player.PlayPause
To bind this to a key you need for example AutoKey.
sudo apt-get install autokey-gtk
Create a new script and use this code:
system.exec_command("dbus-send --print-reply --dest=org.mpris.MediaPlayer2.spotify /org/mpris/MediaPlayer2 org.mpris.MediaPlayer2.Player.PlayPause", getOutput=False)
and bind it to whatever key you want to use.
Of course you can use the same method to create hotkeys for next and previous :)
This command will show you which methods that is available:
mdbus2 org.mpris.MediaPlayer2.spotify /org/mpris/MediaPlayer2
Though, I don't know what will happen if you use both banshee and spotify with the same hotkey!