Prevent user from accidentally navigating away from page

Apologies in advance, as I'm sure that I stuck this in the wrong place.

As a developer I find myself refreshing webpages constantly while developing, and more than once I switch to the wrong tab and hit refresh while listening to Songza. It causes me to lose my place in the playlist (and usually it's in the middle of a great song).

I've added this to prevent users from accidentally navigating away
I'm sure it would be ideal if it only did this while currently playing a song, but I opened this pull request to get some discussion on it.
Let me know if there are any changes you'd like me to make!

![screen shot 2013-10-17 at 5 01 31 pm](https://f.cloud.github.com/assets/566763/1355944/4ef3fbd6-376f-11e3-8054-5abd0d270533.png)

Cheers