Use paths to polymer/SimpleSlider without bower_components

Hi!

First of all - thanks for this element :)

I'm using it with `bower` and this caused me a path problem. I've looked into core-elements source code, and they all are using paths without `bower_components`. As you've put this into `src` directory, i would like to suggest `../../polymer/polymer.html` import (which works well with my configuration).

What do you think?

Best regards,
Kacper.