Add timer() function/event (node timer based) to luacontroller

This adds a timer(<seconds>) function, which causes an event of type
"timer" to be fired after that many seconds has elapsed.

Because it's node timer based, it works properly across server restarts
and block unloading. Thus, simplest example, a blinky plant replacement
with a 10 second period:

if event.type == "program" then
  timer(10)
elseif event.type == "timer" then
  port.a = not port.a
  timer(10)
end