Dynamically load stylesheet based on theme attribute

I think this should fix #10. Loading the styling globally as previously doesn't work anymore in browser that have a real Shadow DOM because the CSS doesn't cross the shadow boundary to the prism elements.

Not sure how legit this approach is, I found it at http://japhr.blogspot.co.uk/2014/06/dynamically-adding-external-styles-to.html

Note that while I upgraded Polymer (I got an error otherwise), I didn't commit the updated content of the `components` directory (why is it checked in?), so you'll need to run `bower install` first.

I suspect more work is needed to bring this custom element up to date (@addyosmani mentions it in other issues), but this at least makes it more usable.