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/

boomerang.js

@@ -259,7 +259,12 @@ boomr = {
 
 		// The developer can override onload by setting autorun to false
 		if(!impl.onloadfired && (!("autorun" in config) || config.autorun !== false)) {
-			impl.addListener(w, "load", BOOMR.page_ready);
+			if("onpagehide" in w) {
+				impl.addListener(w, "pageshow", BOOMR.page_ready);
+			}
+			else {
+				impl.addListener(w, "load", BOOMR.page_ready);
+			}
 		}
 
 		impl.addListener(w, "DOMContentLoaded", function() { impl.fireEvent("dom_loaded"); });
@@ -278,8 +283,13 @@ boomr = {
 
 		impl.addListener(d, "mouseup", impl.onclick_handler);
 
-		// This must be the last one to fire
-		impl.addListener(w, "unload", function() { w=null; });
+		if(!("onpagehide" in w)) {
+			// This must be the last one to fire
+			// We only clear w on browsers that don't support onpagehide because
+			// those that do are new enough to not have memory leak problems of
+			// some older browsers
+			impl.addListener(w, "unload", function() { w=null; });
+		}
 
 		return this;
 	},
@@ -327,22 +337,21 @@ boomr = {
 		// support it.  This allows us to fall back to onunload when onbeforeunload
 		// isn't implemented
 		if(e_name === 'page_unload') {
-			impl.addListener(w, "unload",
-						function() {
+			var unload_handler = function() {
 							if(fn) {
 								fn.call(cb_scope, null, cb_data);
 							}
 							fn=cb_scope=cb_data=null;
-						}
-					);
-			impl.addListener(w, "beforeunload",
-						function() {
-							if(fn) {
-								fn.call(cb_scope, null, cb_data);
-							}
-							fn=cb_scope=cb_data=null;
-						}
-					);
+						};
+			// pagehide is for iOS devices
+			// see http://www.webkit.org/blog/516/webkit-page-cache-ii-the-unload-event/
+			if("onpagehide" in w) {
+				impl.addListener(w, "pagehide", unload_handler);
+			}
+			else {
+				impl.addListener(w, "unload", unload_handler);
+				impl.addListener(w, "beforeunload", unload_handler);
+			}
 		}
 
 		return this;