invokelatest handler

[pfitzseb]

This seems necessary to get handlers to work properly.
With this PR @Ward9250's code in #57 works properly:

using Blink
w = Window()
sleep(5)
body!(w, "My Fancy App!<br/><br/><button id='gobutton'>Click me!</button>")

handle(w, "press") do args
  println(args)
end

@js_ w document.getElementById("gobutton").onclick = () -> Blink.msg("press", 122)
close(w)

Not sure if this is a good idea in general though -- feedback would be appreciated :)