Control Spotify with Media Keys
If you use the Spotify desktop app, the media keys on OS X work by default (sometimes iTunes randomly "retains" the keys: just reboot). For the best results, don't keep iTunes and Spotify open at the same time.

If you use the Spotify Web Player, you can enable the media keys using BeardedSpice, a free application which enables you to control web-based media players using you keyboard media keys. It also works for YouTube, for example.