Implement mouse scrolling for single and double page style navigation

[syang44]

Implemented mouse scrolling for single and double page style navigation. Scroll down navigates to the next pages and vice versa.

Fixes #237

[xgi]

This has an issue where if the page is long / not constrained by window height (i.e. you only see one page but there's still a scrollbar), scrolling will cause it to go to the next page even if you just want to get to the bottom of the image.

Think there'd need to be logic to check if the user is scrolling down while at the bottom (or vice versa). I'm not sure if that's easy to do.