Added params.backbutton

I needed a way to detect if the page was rendered by the user pressing the back button. In case it was, I didn't do anything, but in case it was rendered "normally" (clicking a link), I called `window.scrollTo(0,0);` to scroll the page to the top. This is the way it usually works I believe (on the web without Derby) - the page is scrolled to the top.

So, this feature adds `params.backbutton = true` to the route callbacks `(page, model, params, next)`.
Yes, perhaps, but it seemed like options would always have a value (an object).

called here https://github.com/ilkkah/tracks/blob/feature-backbutton/lib/router.js#L30
and here:
https://github.com/ilkkah/tracks/blob/feature-backbutton/lib/History.js#L46
https://github.com/ilkkah/tracks/blob/feature-backbutton/lib/History.js#L76
https://github.com/ilkkah/tracks/blob/feature-backbutton/lib/History.js#L217
https://github.com/ilkkah/tracks/blob/feature-backbutton/lib/History.js#L228

But I'm not sure if it's is called from other modules. I quickly checked but couldn't find anything.

Should I add the check? It's ok to me. Not sure what your practice is here (how strict).