Fix eventHandlers length was wrong

LWF's eventHandlers length become wrong when use this pattern.

    lwf.addEventHandler('fooEvent', ->
      lwf.removeEventHandler('fooEvent', arguments.callee)
      doSomeThing()
    )

The handler execute logic gets loop count from eventHandlers.length at before execute loop.
But handler function removes handler from eventHandlers in loop.
https://github.com/gree/lwf/blob/master/coffee/js_debug/lwf.js#L6749

This fix creates clone of eventHandlers by `slice(0)`.