Revert "Scroll page to top when prev/next clicked."

This reverts commit 6b71d8de35c2c49dad1dc65fe39f25c5298941be.

It should be done in manageNextPrev() instead.
whenever manageNextPrev() is called, the page should scroll to top

this should also happen in hashchange() so that clicking on the links in the left navigation panel will bring the page to top
Merge pull request #229 from yihui/gh-pages

always scroll to top when location.hash is changed