Fix wheel events; fixes #1048 #1047.

Some time ago the hidden div scroll trick stopped working. But, this went nearly
unnoticed because most browsers are fairly well behaved since WebKit fixed bug
https://bugs.webkit.org/show_bug.cgi?id=40441 back in June 2010. All that
remained broken was Firefox, who adopted the newer W3C "wheel" event standard.
This commit removes the old div hack, and does better support detection to
determine the most standards-based approach to listening for wheel events.