Several zooming tweaks and fixes

Fixes https://github.com/izuzak/atom-pdf-view/issues/22

The factor when zooming out was not being computed correctly before. Also, zooming didn't account for the fact that the padding between the pages is not zoomed. Finally, the computation of the zoomed scrollTop now uses heights of the zoomed pages, instead of computing those heights manually using old page heights. The reason why this helps is that rounding and small division differences add up to a few pixels, which is noticeable. And those differences happen because we're multiplying and dividing by 1.1 and 1.5 in a few places.

Zooming is still done based on the top border of the pdf view, not the center of the page. I like zooming this way, but might re-consider this in the future.