Do not send a beacon on iOS WebViews

It turns out that submitting a hidden form to an anonymous iframe in
a UIWebView on iOS will cause a new Safari window to open with our
beacon URL.  This is not desired behaviour, and until we can figure
out a better way to send a beacon under these conditions, our best bet
is to suppress the beacon.

This is not entirely a problem because any beacon that would have been
sent does not have any timing information (no previous page and no navtiming)

boomerang.js

@@ -634,6 +634,11 @@ boomr = {
 			var input  = document.createElement("input"),
 			    urls = [ impl.beacon_url ];
 
+			// running inside a UIWebView.  Our beacons don't work well here
+			if(navigator.standalone === false) {
+				return;
+			}
+
 			form.method = method;
 			form.id = "beacon_form";