Fix ToC scrolls off screen on articles (close #258)

@sethladd @kwalrath This fixes ToC scrolling has promised in the original issue #258.

A few remarks:
- I had to patch `bootstrap.js`, see the first commit comment for details (fix coming in bs3.1.0),
- I took this opportunity to harmonize things (Polymer ToC now looks like all other ToC, ie background color),
- As a bonus I had a scroll spy to highlight the current section in the ToC (w/ a bold font).

The L&F should now be consistent across `/articles`, `/docs` and `/polymer`