Seperate JavaScript timer implementation from engine.

Currently, JavaScript timer strictly depends on Akka Actor system,
although other parts of JavaScript engine don't depend on Akka.

This patch seperates the timer implementation from engine. After this
patch, who uses BeyondJavaScriptEngine has to implement timer and pass
the timer to engine.
This patch contains timer implementation only for GamePlugin but not for
JavaScriptShellConsole, so not.implemented error is occured when you use
timer in the console.