Add JavaScriptTimerImplementation.

JavaScriptTimerImplementation will be used in the next patch to seperate
timer implementation from engine.
Implement timer for GamePlugin.
Seperate JavaScript timer implementation from engine.

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

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.