Add filter tree functionality

Thanks for the great extension.
I added incremental filter functionality. check the image below.

![screen shot 2014-05-14 at 18 16 07](https://cloud.githubusercontent.com/assets/356330/2969058/b7d60dce-db48-11e3-802e-103814a99951.png)

I'm not a designer nor CSS expert. So if you think my design is ugly please point out to me.