add -100bp, -10bp, +10bp, +100bp nav buttons.

Hi Melissa,

I was reading your blog, which lead me to your repositories, and I saw the little issue about 10 and 100bp navigation buttons. Thought I'd offer this little patch.

![so8wji n](https://cloud.githubusercontent.com/assets/59033/5342344/35fd9d2c-7ecb-11e4-9375-58484ae6b03e.PNG)

Cheers,
--
Andy