Use pageshow/pagehide instead of load/unload

Browsers that use a page cache may not always fire onunload when a user
navigates away from a page but doesn't close the window.  They fire the
pagehide event instead, and the pageshow event when the user returns to
the page.

It's better for us to use these events on browsers that support it.
pageshow/pagehide also fire when onload/unload would normally have
fired.

Thanks to github user @solstice for the info in issue 43:
YahooArchive#43
and the Surfin Safari blog:
http://www.webkit.org/blog/516/webkit-page-cache-ii-the-unload-event/