added helper for angular tree

I use this with spotify and noticed the "flat" way that the playlists are displayed with the restful paths embedded. I created a helper for the [angular-ui-tree](https://github.com/JimLiu/angular-ui-tree) project hoping to get it pulled in. 

This helper creates data from a list of playlists as was in the [demo](http://jimliu.github.io/angular-ui-tree/) 

There is no need to add jquery and, since everything will happen on the client side, it won't bog down raspberry pi users (like myself).