update copyright date
sendBeacon now returns success state
added documentation
updateCookie method returns true/false
no longer need prefix for visibilityState
move methods around for locality
add next url to unload beacon
Refactor to make functions smaller.

Individual blocks of functionality are now in their own methods, and
are called from done() when appropriate.
no need for this comment any more
Add timers to beacon just before beacon is sent

This allows us to add more timers after RT is complete, but before
the beacon is sent.  For example, timers that we only know about after
page load.  The beacon can be delayed by additional plugins, which can
call BOOMR.plugins.RT.setTimer.

Once the before_beacon method is fired, these parameters will be written
to the beacon.
remove unused variables