6641 send beacon

Implemented navigator.sendBeacon as alternative tracking method. (#6641)
I choose to disabled beacon support by default as sendBeacon requests are POST requests (cross
domain awareness). It is easily enabled with `_paq.push(["enableBeacon”]);`.

It uses the same format as bulk requests to avoid posting parameters as forms (resulting in a lot of overhead data).
Ah ok, thank you. I did not notice any JSLint warning on that so I removed it. Maybe I ran it with wrong options. I'll put the line back in.